diff --git a/examples/acp-agent/tests/fixtures/live-mode-switching-2026-07-07.session.jsonl b/examples/acp-agent/tests/fixtures/live-mode-switching-2026-07-07.session.jsonl
index 0852ef7513..10ffb8c507 100644
--- a/examples/acp-agent/tests/fixtures/live-mode-switching-2026-07-07.session.jsonl
+++ b/examples/acp-agent/tests/fixtures/live-mode-switching-2026-07-07.session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"ed16a7e7-a76f-459f-b889-d4c424d66ef6","createdAt":1783421406247,"cwd":"/Users/wwl/workspace/deepseek-harness"}
+{"type":"session","version":0,"id":"ed16a7e7-a76f-459f-b889-d4c424d66ef6","createdAt":1783421406247,"cwd":"/Users/wwl/workspace/deepseek-harness","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783421410388,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783421410388,"data":{"content":[{"type":"text","text":"你好"}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783421410389,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/advanced-toolchain/session.jsonl b/examples/acp-agent/tests/snapshots/advanced-toolchain/session.jsonl
index 39f867984a..46c8e41257 100644
--- a/examples/acp-agent/tests/snapshots/advanced-toolchain/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/advanced-toolchain/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-acp"}
+{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-acp","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783957884479,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783957884479,"data":{"content":[{"type":"text","text":"Run this advanced flow exactly once: mount a no-op Cordis plugin named snapshot-marker; use run_code to inspect the live dynamic mounts through tools.cordis_inspect; delegate once to a direct spawn child; run one workflow that delegates to another spawn child; unmount dyn-1; then reply with exactly ADVANCED_ACP_OK."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783957884486,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/bash-spill/session.jsonl b/examples/acp-agent/tests/snapshots/bash-spill/session.jsonl
index 853b446f81..65f92c4916 100644
--- a/examples/acp-agent/tests/snapshots/bash-spill/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/bash-spill/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Use the bash tool to print a large deterministic output, then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/both-mode-turn/session.jsonl b/examples/acp-agent/tests/snapshots/both-mode-turn/session.jsonl
index 0ee3e67e2b..6b704c5bcd 100644
--- a/examples/acp-agent/tests/snapshots/both-mode-turn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/both-mode-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"bcd7e943-7b84-4264-82d0-f64e50d0d7ce","createdAt":1783611774317,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-52lrTl"}
+{"type":"session","version":0,"id":"bcd7e943-7b84-4264-82d0-f64e50d0d7ce","createdAt":1783611774317,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-52lrTl","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783611774323,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783611774323,"data":{"content":[{"type":"text","text":"Call the run_code tool (NOT the native bash tool directly) with a program that runs exactly `echo BOTH_OK` via tools.bash and returns its output. Then reply with that output only and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783611774324,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/cancel-tool-calls/session.jsonl b/examples/acp-agent/tests/snapshots/cancel-tool-calls/session.jsonl
index e6d18515a6..cdb76be163 100644
--- a/examples/acp-agent/tests/snapshots/cancel-tool-calls/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/cancel-tool-calls/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784437195072,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784437195072,"data":{"content":[{"type":"text","text":"Run two shell commands: wait for cancellation, then write skipped.txt."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784437195076,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/cancel/session.jsonl b/examples/acp-agent/tests/snapshots/cancel/session.jsonl
index 347951db62..44e8ac136c 100644
--- a/examples/acp-agent/tests/snapshots/cancel/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/cancel/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Start a long task; this turn will be cancelled mid-stream."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/code-mode-turn/session.jsonl b/examples/acp-agent/tests/snapshots/code-mode-turn/session.jsonl
index 0fe57055e8..8ee160ba26 100644
--- a/examples/acp-agent/tests/snapshots/code-mode-turn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/code-mode-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"94cd1ae4-e1d1-4ec8-9d27-50a1f849b6b3","createdAt":1783611771392,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-BteTVR"}
+{"type":"session","version":0,"id":"94cd1ae4-e1d1-4ec8-9d27-50a1f849b6b3","createdAt":1783611771392,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-BteTVR","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783611771394,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783611771394,"data":{"content":[{"type":"text","text":"Using ONE run_code program: call the bash tool twice — exactly `echo CODE_ONE` then exactly `echo CODE_TWO` — and return the two outputs joined with a plus sign. Then reply with that joined string only and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783611771396,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/code-mode-workspace-context/session.jsonl b/examples/acp-agent/tests/snapshots/code-mode-workspace-context/session.jsonl
index a20dbcd999..4f72ffa498 100644
--- a/examples/acp-agent/tests/snapshots/code-mode-workspace-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/code-mode-workspace-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"65fbb8a6-624c-4d6a-bf5d-a7a7d14f2b49","createdAt":1783921765266,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-uorU26"}
+{"type":"session","version":0,"id":"65fbb8a6-624c-4d6a-bf5d-a7a7d14f2b49","createdAt":1783921765266,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-uorU26","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783921765269,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783921765269,"data":{"content":[{"type":"text","text":"Using ONE run_code program, call tools.read on nested/task.txt. After the program finishes, answer the workspace handshake question using the newly discovered instructions: What is the Code Mode workspace handshake?"}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783921765275,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/config-options/session.jsonl b/examples/acp-agent/tests/snapshots/config-options/session.jsonl
index a6f73319bc..63f2775383 100644
--- a/examples/acp-agent/tests/snapshots/config-options/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/config-options/session.jsonl
@@ -1 +1 @@
-{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}
+{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0,"delegationDepth":0}
diff --git a/examples/acp-agent/tests/snapshots/cordis-inspect-jsdoc/session.jsonl b/examples/acp-agent/tests/snapshots/cordis-inspect-jsdoc/session.jsonl
index dc9e4a844a..207f8d8cc3 100644
--- a/examples/acp-agent/tests/snapshots/cordis-inspect-jsdoc/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/cordis-inspect-jsdoc/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"22222222-2222-4222-8222-222222222222","createdAt":1783951000000,"cwd":"/tmp/cordis-inspect-jsdoc"}
+{"type":"session","version":0,"id":"22222222-2222-4222-8222-222222222222","createdAt":1783951000000,"cwd":"/tmp/cordis-inspect-jsdoc","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784449176717,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784449176718,"data":{"content":[{"type":"text","text":"Inspect the exact tools service API and tools/pre-execute event with cordis_inspect, then reply with exactly CORDIS_INSPECT_JSDOC_OK."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784449176720,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/error-finish/session.jsonl b/examples/acp-agent/tests/snapshots/error-finish/session.jsonl
index f0ef4267ac..6b6c44ebff 100644
--- a/examples/acp-agent/tests/snapshots/error-finish/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/error-finish/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"This prompt triggers a recorded provider error."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/escalation-approved/session.jsonl b/examples/acp-agent/tests/snapshots/escalation-approved/session.jsonl
index b3931b079b..354a014a63 100644
--- a/examples/acp-agent/tests/snapshots/escalation-approved/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/escalation-approved/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"f3cbd087-fb45-4b32-b0f2-3082d65bfcb4","createdAt":1783860675270,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-cbBLh2"}
+{"type":"session","version":0,"id":"f3cbd087-fb45-4b32-b0f2-3082d65bfcb4","createdAt":1783860675270,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-cbBLh2","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783860675271,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"permission/preset","seq":1,"time":1783962245380,"data":{"preset":"workspace-write"}}
 {"type":"sandbox/mode","seq":2,"time":1784518116517,"data":{"mode":"workspace-write"}}
diff --git a/examples/acp-agent/tests/snapshots/escalation-rejected/session.jsonl b/examples/acp-agent/tests/snapshots/escalation-rejected/session.jsonl
index af1afd3c4a..ff6d1187b3 100644
--- a/examples/acp-agent/tests/snapshots/escalation-rejected/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/escalation-rejected/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"d692fe7f-7079-4ee4-8b06-f44fd026d4ea","createdAt":1783860679475,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-Hn29Od"}
+{"type":"session","version":0,"id":"d692fe7f-7079-4ee4-8b06-f44fd026d4ea","createdAt":1783860679475,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-Hn29Od","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783860679476,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"permission/preset","seq":1,"time":1783962246267,"data":{"preset":"workspace-write"}}
 {"type":"sandbox/mode","seq":2,"time":1784518117237,"data":{"mode":"workspace-write"}}
diff --git a/examples/acp-agent/tests/snapshots/fs-edit/session.jsonl b/examples/acp-agent/tests/snapshots/fs-edit/session.jsonl
index 00587e34c2..25992426b8 100644
--- a/examples/acp-agent/tests/snapshots/fs-edit/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-edit/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"736c4bd8-41bd-43fb-9030-b4df3b2a4f83","createdAt":1783352084735,"cwd":"/tmp/acp-snap-cwd-0BxHdV"}
+{"type":"session","version":0,"id":"736c4bd8-41bd-43fb-9030-b4df3b2a4f83","createdAt":1783352084735,"cwd":"/tmp/acp-snap-cwd-0BxHdV","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352084740,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352084740,"data":{"content":[{"type":"text","text":"First use the read tool to read config.txt in the current directory. Then use the edit tool (NOT bash) to replace the literal text DEBUG with RELEASE in that file. Then reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352084742,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-escalation-approved/session.jsonl b/examples/acp-agent/tests/snapshots/fs-escalation-approved/session.jsonl
index f796b0b473..54601f5354 100644
--- a/examples/acp-agent/tests/snapshots/fs-escalation-approved/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-escalation-approved/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"977a4820-f609-4b48-9039-adcdd921c5fe","createdAt":1784045702340,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-vmEGzd"}
+{"type":"session","version":0,"id":"977a4820-f609-4b48-9039-adcdd921c5fe","createdAt":1784045702340,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-vmEGzd","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784045702342,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"permission/preset","seq":1,"time":1784045702343,"data":{"preset":"workspace-write"}}
 {"type":"sandbox/mode","seq":2,"time":1784045702343,"data":{"mode":"workspace-write"}}
diff --git a/examples/acp-agent/tests/snapshots/fs-policy-reject/session.jsonl b/examples/acp-agent/tests/snapshots/fs-policy-reject/session.jsonl
index 03a64d2e39..66efd5f934 100644
--- a/examples/acp-agent/tests/snapshots/fs-policy-reject/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-policy-reject/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"b3292503-2c3d-4677-804d-1ed6802a4bc5","createdAt":1783611702544,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-QzoqnB"}
+{"type":"session","version":0,"id":"b3292503-2c3d-4677-804d-1ed6802a4bc5","createdAt":1783611702544,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-QzoqnB","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783611702550,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783611702550,"data":{"content":[{"type":"text","text":"Do NOT use the read tool and do NOT use bash or shell commands. Immediately use the edit tool to replace the literal text blue with green in settings.txt in the current directory. Do not read the file first. After the tool result, reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783611702550,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-read-window/session.jsonl b/examples/acp-agent/tests/snapshots/fs-read-window/session.jsonl
index e587011cfd..f0e1f170ac 100644
--- a/examples/acp-agent/tests/snapshots/fs-read-window/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-read-window/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"b5639b9d-99a9-49e4-83da-77e6caa702be","createdAt":1783352099834,"cwd":"/tmp/acp-snap-cwd-N9HCkt"}
+{"type":"session","version":0,"id":"b5639b9d-99a9-49e4-83da-77e6caa702be","createdAt":1783352099834,"cwd":"/tmp/acp-snap-cwd-N9HCkt","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352099838,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352099839,"data":{"content":[{"type":"text","text":"Use the read tool (NOT bash) with offset 5 and limit 4 to read lines 5 through 8 of big.txt in the current directory. Then reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352099840,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-read/session.jsonl b/examples/acp-agent/tests/snapshots/fs-read/session.jsonl
index 81737364f8..5582896174 100644
--- a/examples/acp-agent/tests/snapshots/fs-read/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-read/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"a57f852d-d476-4716-a380-8a1116e4d905","createdAt":1783352072464,"cwd":"/tmp/acp-snap-cwd-PEETkS"}
+{"type":"session","version":0,"id":"a57f852d-d476-4716-a380-8a1116e4d905","createdAt":1783352072464,"cwd":"/tmp/acp-snap-cwd-PEETkS","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352072468,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352072469,"data":{"content":[{"type":"text","text":"Use the read tool (NOT bash) to read the file greeting.txt in the current directory, then reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352072470,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-terminal-card/session.jsonl b/examples/acp-agent/tests/snapshots/fs-terminal-card/session.jsonl
index e53f4b3da3..5c3bba5676 100644
--- a/examples/acp-agent/tests/snapshots/fs-terminal-card/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-terminal-card/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"e128dda9-ed11-4868-8266-0ef90d03c3d6","createdAt":1783352050748,"cwd":"/tmp/acp-snap-cwd-mrFUuk"}
+{"type":"session","version":0,"id":"e128dda9-ed11-4868-8266-0ef90d03c3d6","createdAt":1783352050748,"cwd":"/tmp/acp-snap-cwd-mrFUuk","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352050753,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352050753,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo TERMINAL_OK. Then reply with the single word DONE and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352050755,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-write-overwrite/session.jsonl b/examples/acp-agent/tests/snapshots/fs-write-overwrite/session.jsonl
index 08ec0f1323..ab8b5965ad 100644
--- a/examples/acp-agent/tests/snapshots/fs-write-overwrite/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-write-overwrite/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"e04cc262-6c89-4586-88d7-3e919240d735","createdAt":1783352092215,"cwd":"/tmp/acp-snap-cwd-hH2sGY"}
+{"type":"session","version":0,"id":"e04cc262-6c89-4586-88d7-3e919240d735","createdAt":1783352092215,"cwd":"/tmp/acp-snap-cwd-hH2sGY","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352092220,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352092221,"data":{"content":[{"type":"text","text":"First use the read tool to read data.txt in the current directory. Then use the write tool (NOT bash) to replace its entire contents with exactly the single line: replaced. Then reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352092223,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/fs-write/session.jsonl b/examples/acp-agent/tests/snapshots/fs-write/session.jsonl
index 59ba868817..04fd86da0d 100644
--- a/examples/acp-agent/tests/snapshots/fs-write/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/fs-write/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"fdcab4d0-e5e4-4a06-9195-be8f7049d67e","createdAt":1783352078749,"cwd":"/tmp/acp-snap-cwd-sNvn5N"}
+{"type":"session","version":0,"id":"fdcab4d0-e5e4-4a06-9195-be8f7049d67e","createdAt":1783352078749,"cwd":"/tmp/acp-snap-cwd-sNvn5N","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352078754,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352078754,"data":{"content":[{"type":"text","text":"Use the write tool (NOT bash) to create a file named notes.txt in the current directory containing exactly the single line: hello world. Then reply with exactly the single word DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352078756,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/handshake/session.jsonl b/examples/acp-agent/tests/snapshots/handshake/session.jsonl
index a6f73319bc..63f2775383 100644
--- a/examples/acp-agent/tests/snapshots/handshake/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/handshake/session.jsonl
@@ -1 +1 @@
-{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}
+{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0,"delegationDepth":0}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-posttool-block/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-posttool-block/session.jsonl
index be1d70a853..c362de7a26 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-posttool-block/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-posttool-block/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"669e8682-49fc-4dff-9bc7-6280e283cbe4","createdAt":1783962504097,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-WxJGUY"}
+{"type":"session","version":0,"id":"669e8682-49fc-4dff-9bc7-6280e283cbe4","createdAt":1783962504097,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-WxJGUY","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783962504115,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783962504117,"data":{"content":[{"type":"text","text":"Call the bash tool to run exactly: echo HELLO. If the first tool result is rejected, retry that command once. Quote the final tool result verbatim and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783962504152,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-posttool-context/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-posttool-context/session.jsonl
index 2e167db28c..66d2a6b427 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-posttool-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-posttool-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"0a862642-6652-4916-b88d-b058954ab0c6","createdAt":1783352196657,"cwd":"/tmp/acp-snap-cwd-LEetSL"}
+{"type":"session","version":0,"id":"0a862642-6652-4916-b88d-b058954ab0c6","createdAt":1783352196657,"cwd":"/tmp/acp-snap-cwd-LEetSL","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352196662,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352196662,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo HELLO. Report the tool result you got back verbatim, then stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352196664,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-pretool-ask/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-pretool-ask/session.jsonl
index a1f46a781b..247e13a075 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-pretool-ask/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-pretool-ask/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"f688431c-01a8-4326-a5c5-1b5f0fd08483","createdAt":1783352171511,"cwd":"/tmp/acp-snap-cwd-iKVciS"}
+{"type":"session","version":0,"id":"f688431c-01a8-4326-a5c5-1b5f0fd08483","createdAt":1783352171511,"cwd":"/tmp/acp-snap-cwd-iKVciS","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352171519,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352171520,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo HELLO. Report the tool result you got back verbatim, then stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352171527,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-pretool-deny/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-pretool-deny/session.jsonl
index 9633075e5b..4193c7fe80 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-pretool-deny/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-pretool-deny/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"ff1c1e99-3bd4-4ef8-a954-80d607d628ba","createdAt":1783352165190,"cwd":"/tmp/acp-snap-cwd-wDnkVo"}
+{"type":"session","version":0,"id":"ff1c1e99-3bd4-4ef8-a954-80d607d628ba","createdAt":1783352165190,"cwd":"/tmp/acp-snap-cwd-wDnkVo","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352165195,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352165196,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo HELLO. Report the tool result you got back verbatim, then stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352165198,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-block/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-block/session.jsonl
index b5f81fdaea..dd6745b770 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-block/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-block/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"hook/invoked","seq":1,"time":0,"data":{"turn":1,"point":"UserPromptSubmit","dialect":"claude","handlerId":"claude:UserPromptSubmit:1"}}
 {"type":"hook/result","seq":2,"time":0,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"claude:UserPromptSubmit:1","decision":"block","exitCode":2,"stderrSummary":"blocked by policy hook","durationMs":0}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-context/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-context/session.jsonl
index 886761da18..d2a0806829 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-promptsubmit-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"d03c3a83-1238-4e2e-ad9a-b86a61840a40","createdAt":1783352160541,"cwd":"/tmp/acp-snap-cwd-QUDqlk"}
+{"type":"session","version":0,"id":"d03c3a83-1238-4e2e-ad9a-b86a61840a40","createdAt":1783352160541,"cwd":"/tmp/acp-snap-cwd-QUDqlk","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352160545,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"hook/invoked","seq":1,"time":1783352160546,"data":{"turn":1,"point":"UserPromptSubmit","dialect":"claude","handlerId":"claude:UserPromptSubmit:1"}}
 {"type":"hook/result","seq":2,"time":1783352160564,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"claude:UserPromptSubmit:1","decision":"pass","exitCode":0,"durationMs":17.45639600000004}}
diff --git a/examples/acp-agent/tests/snapshots/hook-cc-stop-continue/session.jsonl b/examples/acp-agent/tests/snapshots/hook-cc-stop-continue/session.jsonl
index baa6a92e9a..c03ee5bd5d 100644
--- a/examples/acp-agent/tests/snapshots/hook-cc-stop-continue/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-cc-stop-continue/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"eda79fbc-8a1b-4226-b74a-f5f297484747","createdAt":1784522140642,"cwd":"/var/folders/4j/54c8wb496zxfrs1ny_21jbb00000gn/T/acp-snap-cwd-r6rWZp"}
+{"type":"session","version":0,"id":"eda79fbc-8a1b-4226-b74a-f5f297484747","createdAt":1784522140642,"cwd":"/var/folders/4j/54c8wb496zxfrs1ny_21jbb00000gn/T/acp-snap-cwd-r6rWZp","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784522140646,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784522140647,"data":{"content":[{"type":"text","text":"Reply with the single word FIRST and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784522140648,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-posttool-block/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-posttool-block/session.jsonl
index 6d08c3ee7d..dc68891c14 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-posttool-block/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-posttool-block/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"01aa6a36-e9c2-42ba-934b-30bec80a1658","createdAt":1783986962232,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-x67BsP"}
+{"type":"session","version":0,"id":"01aa6a36-e9c2-42ba-934b-30bec80a1658","createdAt":1783986962232,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-x67BsP","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783986962235,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783986962235,"data":{"content":[{"type":"text","text":"Call the bash tool exactly once to run: echo HELLO. Whatever tool result comes back, quote it verbatim and stop without calling another tool."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783986962240,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-posttool-context/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-posttool-context/session.jsonl
index d60858ee0a..b7e97b3a65 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-posttool-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-posttool-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"39d8aabe-6457-4a0e-83b7-ee33125a3666","createdAt":1783352228436,"cwd":"/tmp/acp-snap-cwd-VGFtPi"}
+{"type":"session","version":0,"id":"39d8aabe-6457-4a0e-83b7-ee33125a3666","createdAt":1783352228436,"cwd":"/tmp/acp-snap-cwd-VGFtPi","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352228441,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352228442,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo HELLO. Report the tool result you got back verbatim, then stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352228443,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-pretool-block/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-pretool-block/session.jsonl
index 812998aad5..c2675ae3af 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-pretool-block/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-pretool-block/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"57a74aed-99fc-43bc-a875-6dddebf64d69","createdAt":1783352214599,"cwd":"/tmp/acp-snap-cwd-7Hbu0m"}
+{"type":"session","version":0,"id":"57a74aed-99fc-43bc-a875-6dddebf64d69","createdAt":1783352214599,"cwd":"/tmp/acp-snap-cwd-7Hbu0m","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352214604,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352214605,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo HELLO. Report the tool result you got back verbatim, then stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352214607,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-block/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-block/session.jsonl
index bc9144f980..126a761309 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-block/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-block/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"hook/invoked","seq":1,"time":0,"data":{"turn":1,"point":"UserPromptSubmit","dialect":"codex","handlerId":"codex:UserPromptSubmit:1"}}
 {"type":"hook/result","seq":2,"time":0,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"codex:UserPromptSubmit:1","decision":"block","exitCode":2,"stderrSummary":"blocked by codex policy hook","durationMs":0}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-context/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-context/session.jsonl
index 1c710ff8e1..ad7538cba9 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-promptsubmit-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"0bebc0f4-a089-4fde-9b6e-db9532cfd4de","createdAt":1783352209682,"cwd":"/tmp/acp-snap-cwd-aopaZV"}
+{"type":"session","version":0,"id":"0bebc0f4-a089-4fde-9b6e-db9532cfd4de","createdAt":1783352209682,"cwd":"/tmp/acp-snap-cwd-aopaZV","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352209686,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"hook/invoked","seq":1,"time":1783352209687,"data":{"turn":1,"point":"UserPromptSubmit","dialect":"codex","handlerId":"codex:UserPromptSubmit:1"}}
 {"type":"hook/result","seq":2,"time":1783352209706,"data":{"turn":1,"point":"UserPromptSubmit","handlerId":"codex:UserPromptSubmit:1","decision":"pass","exitCode":0,"durationMs":19.49695100000008}}
diff --git a/examples/acp-agent/tests/snapshots/hook-codex-stop-continue/session.jsonl b/examples/acp-agent/tests/snapshots/hook-codex-stop-continue/session.jsonl
index cee758076c..5cf14d9b49 100644
--- a/examples/acp-agent/tests/snapshots/hook-codex-stop-continue/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/hook-codex-stop-continue/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"eb17be12-ca8c-46c8-b500-0977e8400208","createdAt":1784522152392,"cwd":"/var/folders/4j/54c8wb496zxfrs1ny_21jbb00000gn/T/acp-snap-cwd-ESgqLu"}
+{"type":"session","version":0,"id":"eb17be12-ca8c-46c8-b500-0977e8400208","createdAt":1784522152392,"cwd":"/var/folders/4j/54c8wb496zxfrs1ny_21jbb00000gn/T/acp-snap-cwd-ESgqLu","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784522152397,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784522152397,"data":{"content":[{"type":"text","text":"Reply with the single word FIRST and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784522152399,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/model-switching/session.jsonl b/examples/acp-agent/tests/snapshots/model-switching/session.jsonl
index 1f04eca07d..ddc592bbb6 100644
--- a/examples/acp-agent/tests/snapshots/model-switching/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/model-switching/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"622d16ce-0a94-476b-97a4-26dad50b1fbf","createdAt":1784086275585,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-Cwf7Bh"}
+{"type":"session","version":0,"id":"622d16ce-0a94-476b-97a4-26dad50b1fbf","createdAt":1784086275585,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-Cwf7Bh","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784086275588,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784086275588,"data":{"content":[{"type":"text","text":"Without using tools, reply with exactly FLASH and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784086275590,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/multi-turn/session.jsonl b/examples/acp-agent/tests/snapshots/multi-turn/session.jsonl
index cd32072eaa..83ccf18a3f 100644
--- a/examples/acp-agent/tests/snapshots/multi-turn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/multi-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"228b7b82-84ed-49b7-a567-981c03b28c77","createdAt":1783352113760,"cwd":"/tmp/acp-snap-cwd-aN2GRR"}
+{"type":"session","version":0,"id":"228b7b82-84ed-49b7-a567-981c03b28c77","createdAt":1783352113760,"cwd":"/tmp/acp-snap-cwd-aN2GRR","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352113765,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352113765,"data":{"content":[{"type":"text","text":"Reply with exactly the word: ONE. No tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352113767,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/parallel-tool-calls/session.jsonl b/examples/acp-agent/tests/snapshots/parallel-tool-calls/session.jsonl
index 81503dc4cf..e83f0cd59c 100644
--- a/examples/acp-agent/tests/snapshots/parallel-tool-calls/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/parallel-tool-calls/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Use the read tool twice in the same assistant message: read a.txt and b.txt. Then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/permission-switching/session.jsonl b/examples/acp-agent/tests/snapshots/permission-switching/session.jsonl
index 43fc43d979..ff44da7e1d 100644
--- a/examples/acp-agent/tests/snapshots/permission-switching/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/permission-switching/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"df041acb-2f14-4d5f-b6e2-2fb6b9eb6427","createdAt":1783860666204,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-4oJKT4"}
+{"type":"session","version":0,"id":"df041acb-2f14-4d5f-b6e2-2fb6b9eb6427","createdAt":1783860666204,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-4oJKT4","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783860666206,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"permission/preset","seq":1,"time":1783962244578,"data":{"preset":"workspace-write"}}
 {"type":"sandbox/mode","seq":2,"time":1784518115721,"data":{"mode":"workspace-write"}}
diff --git a/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl b/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl
index a6f73319bc..63f2775383 100644
--- a/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/reject-extra-dirs/session.jsonl
@@ -1 +1 @@
-{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0}
+{"type":"session","version":0,"id":"00000000-0000-0000-0000-000000000000","createdAt":0,"delegationDepth":0}
diff --git a/examples/acp-agent/tests/snapshots/repeat-tool-guard/session.jsonl b/examples/acp-agent/tests/snapshots/repeat-tool-guard/session.jsonl
index 20d18973d9..c77e0ef38d 100644
--- a/examples/acp-agent/tests/snapshots/repeat-tool-guard/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/repeat-tool-guard/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Write the todo list 'watch the kettle boil' five times in a row without changing it, then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/skill-load/session.jsonl b/examples/acp-agent/tests/snapshots/skill-load/session.jsonl
index 0fe1a5152c..1a9704954a 100644
--- a/examples/acp-agent/tests/snapshots/skill-load/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/skill-load/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"9eb4181f-2d05-49d3-98fc-3711fe2f5664","createdAt":1783654655599,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-DhYwNW"}
+{"type":"session","version":0,"id":"9eb4181f-2d05-49d3-98fc-3711fe2f5664","createdAt":1783654655599,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-DhYwNW","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783654655602,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783654655603,"data":{"content":[{"type":"text","text":"Load the snapshot-skill skill with the skill tool, then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783654655608,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-fork/session.jsonl b/examples/acp-agent/tests/snapshots/subagent-fork/session.jsonl
index 78846dd11f..5fb8854903 100644
--- a/examples/acp-agent/tests/snapshots/subagent-fork/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/subagent-fork/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"19a0ab16-a36d-49c8-bac2-c1b2208844ad","createdAt":1784451778257,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-CuULie"}
+{"type":"session","version":0,"id":"19a0ab16-a36d-49c8-bac2-c1b2208844ad","createdAt":1784451778257,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-CuULie","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784451778261,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784451778262,"data":{"content":[{"type":"text","text":"Remember this fact for later: the project codeword is MARMALADE. Reply with the single word OK and stop. Do not use any tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784451778263,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-mixed/session.jsonl b/examples/acp-agent/tests/snapshots/subagent-mixed/session.jsonl
index ee9b134de6..e5029f4248 100644
--- a/examples/acp-agent/tests/snapshots/subagent-mixed/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/subagent-mixed/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"91b46b45-a870-42dc-9314-be4ceeb9c3f3","createdAt":1784451785949,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-vBPxcm"}
+{"type":"session","version":0,"id":"91b46b45-a870-42dc-9314-be4ceeb9c3f3","createdAt":1784451785949,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-vBPxcm","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784451785951,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784451785952,"data":{"content":[{"type":"text","text":"Remember this fact for later: the project codeword is SAFFRON. Reply with the single word OK and stop. Do not use any tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784451785955,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-multi/session.jsonl b/examples/acp-agent/tests/snapshots/subagent-multi/session.jsonl
index 9649a197bd..75a7460776 100644
--- a/examples/acp-agent/tests/snapshots/subagent-multi/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/subagent-multi/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"23127c8b-3c39-4dca-8cb6-8111f50bd23f","createdAt":1784451767994,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-ogFsTm"}
+{"type":"session","version":0,"id":"23127c8b-3c39-4dca-8cb6-8111f50bd23f","createdAt":1784451767994,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-ogFsTm","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784451767996,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784451767996,"data":{"content":[{"type":"text","text":"Use the subagent tool TWICE, once at a time, to delegate two subtasks to child agents. First subtask: 'Reply with exactly the word ALPHA and nothing else.' Second subtask (after the first returns): 'Reply with exactly the word BETA and nothing else.' After both subagents return, reply with the single word PARENT_DONE and stop. Do not use the bash tool."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784451768000,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/subagent-spawn/session.jsonl b/examples/acp-agent/tests/snapshots/subagent-spawn/session.jsonl
index 5dddaa522f..c46bf5237b 100644
--- a/examples/acp-agent/tests/snapshots/subagent-spawn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/subagent-spawn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"5ab41657-0a0f-4317-88fe-451c5197cdb4","createdAt":1784451761926,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-ErhW9C"}
+{"type":"session","version":0,"id":"5ab41657-0a0f-4317-88fe-451c5197cdb4","createdAt":1784451761926,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-ErhW9C","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784451761930,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784451761930,"data":{"content":[{"type":"text","text":"Use the subagent tool exactly once to delegate this subtask to a child agent: 'Reply with exactly the word CHILD_OK and nothing else.' After the subagent returns, reply with the single word PARENT_DONE and stop. Do not use the bash tool."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784451761932,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/text-turn/session.jsonl b/examples/acp-agent/tests/snapshots/text-turn/session.jsonl
index 04ac2f9794..1f2209ca19 100644
--- a/examples/acp-agent/tests/snapshots/text-turn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/text-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"539aa64c-7f37-40ff-abd8-ed45b717be1b","createdAt":1783600629539,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-ka5r8w"}
+{"type":"session","version":0,"id":"539aa64c-7f37-40ff-abd8-ed45b717be1b","createdAt":1783600629539,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-ka5r8w","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783600629541,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783600629541,"data":{"content":[{"type":"text","text":"Reply with exactly the word: PONG. Do not use any tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783600629542,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/todo-plan/session.jsonl b/examples/acp-agent/tests/snapshots/todo-plan/session.jsonl
index 909afc44cd..cea8a4fa88 100644
--- a/examples/acp-agent/tests/snapshots/todo-plan/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/todo-plan/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"b0f1f758-dcf0-474e-851d-e62c11ec0a09","createdAt":1783352057652,"cwd":"/tmp/acp-snap-cwd-AYilT7"}
+{"type":"session","version":0,"id":"b0f1f758-dcf0-474e-851d-e62c11ec0a09","createdAt":1783352057652,"cwd":"/tmp/acp-snap-cwd-AYilT7","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352057655,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352057655,"data":{"content":[{"type":"text","text":"Use the todo_write tool to record a plan with exactly three todos: \"read the code\" (in_progress), \"write the fix\" (pending), \"run the tests\" (pending). Send all three in one todo_write call. Then reply with the single word DONE and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352057657,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/tool-call-turn/session.jsonl b/examples/acp-agent/tests/snapshots/tool-call-turn/session.jsonl
index f631c9bf54..3b0e93b464 100644
--- a/examples/acp-agent/tests/snapshots/tool-call-turn/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/tool-call-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"e9421ff4-baae-4807-a7ea-fd8a65f2c897","createdAt":1783352044766,"cwd":"/tmp/acp-snap-cwd-OwUkBh"}
+{"type":"session","version":0,"id":"e9421ff4-baae-4807-a7ea-fd8a65f2c897","createdAt":1783352044766,"cwd":"/tmp/acp-snap-cwd-OwUkBh","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352044771,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352044771,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo SNAPSHOT_OK. Then reply with the single word DONE and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352044773,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/workflow-run/session.jsonl b/examples/acp-agent/tests/snapshots/workflow-run/session.jsonl
index 55b622307d..a520e28d08 100644
--- a/examples/acp-agent/tests/snapshots/workflow-run/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/workflow-run/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"6789922c-5a8c-4141-8336-0f9b0809bb17","createdAt":1784451802866,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-Uzz8l5"}
+{"type":"session","version":0,"id":"6789922c-5a8c-4141-8336-0f9b0809bb17","createdAt":1784451802866,"cwd":"/var/folders/_g/59jgff8x2gqd39f5vy1wnbfc0000gn/T/acp-snap-cwd-Uzz8l5","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1784451802869,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1784451802870,"data":{"content":[{"type":"text","text":"Use the workflow tool exactly once, with args omitted, meta set to { \"name\": \"snapshot-flow\", \"description\": \"one child for the snapshot\" }, and this EXACT script body (copy it verbatim):\nphase('Run')\nconst reply = await agent('Reply with exactly the word WF_CHILD_OK and nothing else.')\nreturn { reply }\nAfter the workflow returns, reply with the single word WORKFLOW_DONE and stop. Do not use any other tool."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1784451802872,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/workspace-context/session.jsonl b/examples/acp-agent/tests/snapshots/workspace-context/session.jsonl
index f50e124b32..9e08a7800b 100644
--- a/examples/acp-agent/tests/snapshots/workspace-context/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/workspace-context/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783778297065,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783778297066,"data":{"content":[{"type":"text","text":"Read nested/task.txt with the read tool, then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783778297069,"data":{"turn":1,"step":1}}
diff --git a/examples/acp-agent/tests/snapshots/workspace-edit/session.jsonl b/examples/acp-agent/tests/snapshots/workspace-edit/session.jsonl
index 6b9b03a95e..4a5b37a374 100644
--- a/examples/acp-agent/tests/snapshots/workspace-edit/session.jsonl
+++ b/examples/acp-agent/tests/snapshots/workspace-edit/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"48aca674-000a-4583-810b-01f8785cef13","createdAt":1783352264076,"cwd":"/tmp/acp-snap-cwd-rxbEpP"}
+{"type":"session","version":0,"id":"48aca674-000a-4583-810b-01f8785cef13","createdAt":1783352264076,"cwd":"/tmp/acp-snap-cwd-rxbEpP","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352264080,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352264081,"data":{"content":[{"type":"text","text":"A file named greeting.txt in the current directory contains one word. Use the bash tool to append a second line containing the word WORLD to it (so it has two lines), then read the file back with `cat greeting.txt` to confirm, and reply with the single word DONE. Use a single bash call per action."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352264082,"data":{"turn":1,"step":1}}
diff --git a/examples/headless-agent/tests/snapshots/advanced-toolchain/session.jsonl b/examples/headless-agent/tests/snapshots/advanced-toolchain/session.jsonl
index e3849110ce..045d7879df 100644
--- a/examples/headless-agent/tests/snapshots/advanced-toolchain/session.jsonl
+++ b/examples/headless-agent/tests/snapshots/advanced-toolchain/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-headless"}
+{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-headless","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783957884479,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783957884479,"data":{"content":[{"type":"text","text":"Run this advanced flow exactly once: mount a no-op Cordis plugin named snapshot-marker; use run_code to inspect the live dynamic mounts through tools.cordis_inspect; delegate once to a direct spawn child; run one workflow that delegates to another spawn child; unmount dyn-1; then reply with exactly ADVANCED_HEADLESS_OK."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783957884486,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/bash-terminal-card/session.jsonl b/examples/tui-agent/tests/snapshots/bash-terminal-card/session.jsonl
index e53f4b3da3..5c3bba5676 100644
--- a/examples/tui-agent/tests/snapshots/bash-terminal-card/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/bash-terminal-card/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"e128dda9-ed11-4868-8266-0ef90d03c3d6","createdAt":1783352050748,"cwd":"/tmp/acp-snap-cwd-mrFUuk"}
+{"type":"session","version":0,"id":"e128dda9-ed11-4868-8266-0ef90d03c3d6","createdAt":1783352050748,"cwd":"/tmp/acp-snap-cwd-mrFUuk","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352050753,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352050753,"data":{"content":[{"type":"text","text":"Use the bash tool to run exactly: echo TERMINAL_OK. Then reply with the single word DONE and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352050755,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/code-mode/session.jsonl b/examples/tui-agent/tests/snapshots/code-mode/session.jsonl
index 0fe57055e8..8ee160ba26 100644
--- a/examples/tui-agent/tests/snapshots/code-mode/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/code-mode/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"94cd1ae4-e1d1-4ec8-9d27-50a1f849b6b3","createdAt":1783611771392,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-BteTVR"}
+{"type":"session","version":0,"id":"94cd1ae4-e1d1-4ec8-9d27-50a1f849b6b3","createdAt":1783611771392,"cwd":"/var/folders/2c/psb0_fmx7hbgz558xjt_f0l00000gn/T/acp-snap-cwd-BteTVR","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783611771394,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783611771394,"data":{"content":[{"type":"text","text":"Using ONE run_code program: call the bash tool twice — exactly `echo CODE_ONE` then exactly `echo CODE_TWO` — and return the two outputs joined with a plus sign. Then reply with that joined string only and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783611771396,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.1.jsonl b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.1.jsonl
index 25a6f76411..26519d458e 100644
--- a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.1.jsonl
+++ b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.1.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"22222222-2222-4222-8222-222222222222","createdAt":1783950001000,"cwd":"/tmp/advanced-acp","parentSession":"11111111-1111-4111-8111-111111111111"}
+{"type":"session","version":0,"id":"22222222-2222-4222-8222-222222222222","createdAt":1783950001000,"cwd":"/tmp/advanced-acp","parentSession":"11111111-1111-4111-8111-111111111111","delegationDepth":1}
 {"type":"turn/start","seq":0,"time":1783957884563,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783957884563,"data":{"content":[{"type":"text","text":"Reply with exactly DIRECT_CHILD_OK and nothing else."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783957884564,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.2.jsonl b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.2.jsonl
index 45d9043a4a..9daa8958dc 100644
--- a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.2.jsonl
+++ b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.2.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"33333333-3333-4333-8333-333333333333","createdAt":1783950002000,"cwd":"/tmp/advanced-acp","parentSession":"11111111-1111-4111-8111-111111111111"}
+{"type":"session","version":0,"id":"33333333-3333-4333-8333-333333333333","createdAt":1783950002000,"cwd":"/tmp/advanced-acp","parentSession":"11111111-1111-4111-8111-111111111111","delegationDepth":1}
 {"type":"turn/start","seq":0,"time":1783957884700,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783957884700,"data":{"content":[{"type":"text","text":"Reply with exactly WORKFLOW_CHILD_OK and nothing else."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783957884700,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.jsonl b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.jsonl
index 39f867984a..46c8e41257 100644
--- a/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/cordis-dynamic-toolchain/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-acp"}
+{"type":"session","version":0,"id":"11111111-1111-4111-8111-111111111111","createdAt":1783950000000,"cwd":"/tmp/advanced-acp","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783957884479,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783957884479,"data":{"content":[{"type":"text","text":"Run this advanced flow exactly once: mount a no-op Cordis plugin named snapshot-marker; use run_code to inspect the live dynamic mounts through tools.cordis_inspect; delegate once to a direct spawn child; run one workflow that delegates to another spawn child; unmount dyn-1; then reply with exactly ADVANCED_ACP_OK."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783957884486,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/dynamic-workflow/session.1.jsonl b/examples/tui-agent/tests/snapshots/dynamic-workflow/session.1.jsonl
index 3d89428bbd..eb8d9ed63e 100644
--- a/examples/tui-agent/tests/snapshots/dynamic-workflow/session.1.jsonl
+++ b/examples/tui-agent/tests/snapshots/dynamic-workflow/session.1.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"583a4db2-3350-436c-b4a5-5615fd159052","createdAt":1783600636316,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-vdJYjz","parentSession":"3fd7d599-56b1-493a-930d-f1fc5e1556e8"}
+{"type":"session","version":0,"id":"583a4db2-3350-436c-b4a5-5615fd159052","createdAt":1783600636316,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-vdJYjz","parentSession":"3fd7d599-56b1-493a-930d-f1fc5e1556e8","delegationDepth":1}
 {"type":"turn/start","seq":0,"time":1783600636316,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783600636316,"data":{"content":[{"type":"text","text":"Reply with exactly the word WF_CHILD_OK and nothing else."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783600636316,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/dynamic-workflow/session.jsonl b/examples/tui-agent/tests/snapshots/dynamic-workflow/session.jsonl
index 3e0ae3da73..20f4e296cd 100644
--- a/examples/tui-agent/tests/snapshots/dynamic-workflow/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/dynamic-workflow/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"3fd7d599-56b1-493a-930d-f1fc5e1556e8","createdAt":1783600631835,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-vdJYjz"}
+{"type":"session","version":0,"id":"3fd7d599-56b1-493a-930d-f1fc5e1556e8","createdAt":1783600631835,"cwd":"/var/folders/bn/vj1dvck95yd5jh3x4wskflxm0000gn/T/acp-snap-cwd-vdJYjz","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783600631838,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783600631838,"data":{"content":[{"type":"text","text":"Use the workflow tool exactly once, with args omitted, meta set to { \"name\": \"snapshot-flow\", \"description\": \"one child for the snapshot\" }, and this EXACT script body (copy it verbatim):\nphase('Run')\nconst reply = await agent('Reply with exactly the word WF_CHILD_OK and nothing else.')\nreturn { reply }\nAfter the workflow returns, reply with the single word WORKFLOW_DONE and stop. Do not use any other tool."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783600631839,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/multi-turn-conversation/session.jsonl b/examples/tui-agent/tests/snapshots/multi-turn-conversation/session.jsonl
index cd32072eaa..83ccf18a3f 100644
--- a/examples/tui-agent/tests/snapshots/multi-turn-conversation/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/multi-turn-conversation/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"228b7b82-84ed-49b7-a567-981c03b28c77","createdAt":1783352113760,"cwd":"/tmp/acp-snap-cwd-aN2GRR"}
+{"type":"session","version":0,"id":"228b7b82-84ed-49b7-a567-981c03b28c77","createdAt":1783352113760,"cwd":"/tmp/acp-snap-cwd-aN2GRR","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352113765,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352113765,"data":{"content":[{"type":"text","text":"Reply with exactly the word: ONE. No tools."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352113767,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/parallel-file-reads/session.jsonl b/examples/tui-agent/tests/snapshots/parallel-file-reads/session.jsonl
index 81503dc4cf..e83f0cd59c 100644
--- a/examples/tui-agent/tests/snapshots/parallel-file-reads/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/parallel-file-reads/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","version":0,"id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":0,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"Use the read tool twice in the same assistant message: read a.txt and b.txt. Then reply DONE."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
diff --git a/examples/tui-agent/tests/snapshots/todo-plan/session.jsonl b/examples/tui-agent/tests/snapshots/todo-plan/session.jsonl
index 909afc44cd..cea8a4fa88 100644
--- a/examples/tui-agent/tests/snapshots/todo-plan/session.jsonl
+++ b/examples/tui-agent/tests/snapshots/todo-plan/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","version":0,"id":"b0f1f758-dcf0-474e-851d-e62c11ec0a09","createdAt":1783352057652,"cwd":"/tmp/acp-snap-cwd-AYilT7"}
+{"type":"session","version":0,"id":"b0f1f758-dcf0-474e-851d-e62c11ec0a09","createdAt":1783352057652,"cwd":"/tmp/acp-snap-cwd-AYilT7","delegationDepth":0}
 {"type":"turn/start","seq":0,"time":1783352057655,"data":{"turn":1,"trigger":{"kind":"message","source":{"kind":"user"}}}}
 {"type":"user/message","seq":1,"time":1783352057655,"data":{"content":[{"type":"text","text":"Use the todo_write tool to record a plan with exactly three todos: \"read the code\" (in_progress), \"write the fix\" (pending), \"run the tests\" (pending). Send all three in one todo_write call. Then reply with the single word DONE and stop."}],"source":{"kind":"user"}},"surfaceOp":"append"}
 {"type":"step/start","seq":2,"time":1783352057657,"data":{"turn":1,"step":1}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.1.jsonl b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.1.jsonl
index 1caf2610b3..4fa81014ae 100644
--- a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.1.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.1.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"abababab-cdcd-4efe-8ada-badabadabada","createdAt":800,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-KBQJbW","parentSession":"f6fa7fcf-dd9c-4b39-8815-b25ddcebfd88"}
+{"type":"session","id":"abababab-cdcd-4efe-8ada-badabadabada","createdAt":800,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-KBQJbW","parentSession":"f6fa7fcf-dd9c-4b39-8815-b25ddcebfd88","delegationDepth":1}
 {"type":"request/header","seq":0,"time":2,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.jsonl
index a2beac360d..e972a78d8e 100644
--- a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-child/session.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"f6fa7fcf-dd9c-4b39-8815-b25ddcebfd88","createdAt":700,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-KBQJbW"}
+{"type":"session","id":"f6fa7fcf-dd9c-4b39-8815-b25ddcebfd88","createdAt":700,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-KBQJbW","delegationDepth":0}
 {"type":"request/header","seq":0,"time":3,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/session.jsonl
index e9dd3fb16c..40bb4d37dc 100644
--- a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-pin/session.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"ccdc749f-56f3-4267-9750-598b5c60b7b2","createdAt":600,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-nOQ4Gy"}
+{"type":"session","id":"ccdc749f-56f3-4267-9750-598b5c60b7b2","createdAt":600,"cwd":"/var/folders/2g/b32ct0qn1d728l_v6tdkjytr0000gn/T/acp-snap-cwd-nOQ4Gy","delegationDepth":0}
 {"type":"request/header","seq":0,"time":4,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-skip/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-skip/session.jsonl
index 104f2a0df2..035d1353b7 100644
--- a/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-skip/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/record-suite/rec-skip/session.jsonl
@@ -1 +1 @@
-{"type":"session","id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/authored-error/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/authored-error/session.jsonl
index 36991a214e..a202c7141d 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/authored-error/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/authored-error/session.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"44444444-3333-4222-8111-000000000000","createdAt":17,"cwd":"/rec/authored-cwd"}
+{"type":"session","id":"44444444-3333-4222-8111-000000000000","createdAt":17,"cwd":"/rec/authored-cwd","delegationDepth":0}
 {"type":"turn/end","seq":1,"time":17,"data":{"error":"model exploded"}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/blocked-log/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/blocked-log/session.jsonl
index 6d8474812d..75eb18fa71 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/blocked-log/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/blocked-log/session.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"99999999-8888-4777-8666-555555555555","createdAt":13,"cwd":"/rec/blocked-cwd"}
+{"type":"session","id":"99999999-8888-4777-8666-555555555555","createdAt":13,"cwd":"/rec/blocked-cwd","delegationDepth":0}
 {"type":"hook/result","seq":1,"time":13,"data":{"decision":"block","durationMs":99}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/no-model/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/no-model/session.jsonl
index 104f2a0df2..035d1353b7 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/no-model/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/no-model/session.jsonl
@@ -1 +1 @@
-{"type":"session","id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}"}
+{"type":"session","id":"{{sessionId}}","createdAt":0,"cwd":"{{cwd}}","delegationDepth":0}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl
index 54b64b59a3..693e728d2c 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/pin-turn/session.jsonl
@@ -1,4 +1,4 @@
-{"type":"session","id":"12121212-3434-4545-8686-787878787878","createdAt":7,"cwd":"/rec/pin-cwd"}
+{"type":"session","id":"12121212-3434-4545-8686-787878787878","createdAt":7,"cwd":"/rec/pin-cwd","delegationDepth":0}
 {"type":"request/header","seq":0,"time":7,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
 {"type":"request/header","seq":1,"time":7,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"change"}}
 {"type":"turn/start","seq":2,"time":7,"data":{"turn":1}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.1.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.1.jsonl
index a844f891fc..5fb01dbe8a 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.1.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.1.jsonl
@@ -1,2 +1,2 @@
-{"type":"session","id":"eeeeeeee-1111-4222-8333-444444444444","createdAt":12,"cwd":"/rec/plain-cwd","parentSession":"56565656-7878-4989-8a9a-9b9b9b9b9b9b"}
+{"type":"session","id":"eeeeeeee-1111-4222-8333-444444444444","createdAt":12,"cwd":"/rec/plain-cwd","parentSession":"56565656-7878-4989-8a9a-9b9b9b9b9b9b","delegationDepth":1}
 {"type":"request/header","seq":0,"time":12,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
diff --git a/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.jsonl b/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.jsonl
index 744998f959..6bdf784ce2 100644
--- a/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.jsonl
+++ b/packages/support/acp-snapshot/tests/fixtures/suite/plain-turn/session.jsonl
@@ -1,3 +1,3 @@
-{"type":"session","id":"56565656-7878-4989-8a9a-9b9b9b9b9b9b","createdAt":11,"cwd":"/rec/plain-cwd"}
+{"type":"session","id":"56565656-7878-4989-8a9a-9b9b9b9b9b9b","createdAt":11,"cwd":"/rec/plain-cwd","delegationDepth":0}
 {"type":"request/header","seq":0,"time":11,"data":{"header":{"config":{"model":"fake"},"system":"{{system}}","tools":"{{tools}}"},"reason":"initial"}}
 {"type":"assistant/chunk","seq":1,"time":11,"data":{"turn":1,"step":1,"chunk":{"type":"text-delta","index":0,"text":"hi"}}}