diff --git a/docs/architecture.md b/docs/architecture.md
index 2b05e613b1..fb2d594283 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -53,6 +53,7 @@ Dependency rule: **extension** plugins depend on interface packages, never on `d
 | `ctx.agents` | `AgentRegistry` | dsh-agent | live `Agent` handles + the create/resume factory seam (returns an `AgentHandle` = `{ agent, dispose() }` for owned per-agent teardown) |
 | `ctx.agentLoop` | `AgentLoop` | dsh-agent-loop | creates `ReactLoopAgent`s and drives their loops |
 | `ctx.bash` | `BashExecutor` (abstract) | dsh-bash | bash execution seam: foreground runs + background tasks |
+| `ctx.compact` | `CompactService` (abstract) | dsh-compact | compaction seam: decide when history is too large, summarize an older range into a single surface node |
 
 All registrations (`registerAdapter`, `section`, `tools`, `register`, …) go through `ctx.effect()` and return disposers, so plugin hot-reload (vendored HMR) and fiber disposal clean up automatically.
 
@@ -191,7 +192,7 @@ Every MVP feature (including the TODO-marked ones), with the mechanism that impl
 | `/loop` | on `agent/turn-end`, `send()` the next iteration; or force-continue |
 | Dynamic workflow | orchestrator plugin on `agent/turn-end` / `agent/step-end` driving `send`/`steer` (+ sub-agents later) |
 | Queued + steering messages | core `Agent.send()` / `Agent.steer()` |
-| Context compaction (auto + manual) | wrap `agent/request`: measure tokens, rewrite `req.messages`, append merged `compaction/*` session events; manual = a command plugin invoking the same routine |
+| Context compaction (auto + manual) | the `ctx.compact` seam ([dsh-compact](../packages/compact/compact)): a backend summarizes an older surface range into a single `user/message` `replace` op, bracketed by log-only `compact/*` events; auto = check token pressure at turn boundaries, manual = a `/compact` tool. See the [compaction capability-seam RFC](rfc/proposed/feature/2026-06-18-compaction-capability-seam.md) |
 | System prompt configurability | `ctx.systemPrompt.section()` with ordering |
 | AGENTS.md (root) | a section provider reading the file |
 | AGENTS.md (subdir, on-touch) + file-change notices | `agent.inject()` from a watcher / tool-result listener |
diff --git a/packages/README.md b/packages/README.md
index f39d7b70a1..78eefbac9d 100644
--- a/packages/README.md
+++ b/packages/README.md
@@ -11,6 +11,7 @@ Packages are grouped by modular role at `packages/<group>/<pkg>/`. The group dir
 | [`core/`](core/README.md) | Product API spine: session, system-prompt, tools, agent, and the concrete loop | Product — stable surface |
 | [`llm/`](llm/README.md) | LLM capability family: the abstract service + provider adapters | Product — stable surface |
 | [`bash/`](bash/README.md) | Bash capability family: the executor seam, a local impl, and the model-facing tool | Product — stable surface |
+| [`compact/`](compact/README.md) | Compaction capability family: the abstract seam (backend + tool deferred) | Product — stable surface |
 | [`session-persistence/`](session-persistence/README.md) | Persistence capability family: the seam + JSONL/SQLite backends | Product — stable surface |
 | [`ui/`](ui/README.md) | Editor/client integration surfaces (the ACP bridge) | Product — stable surface |
 | [`support/`](support/README.md) | Dev/test/example infrastructure (invariants, stdio UI, replay adapter) | Support — lower compatibility expectations |
@@ -27,6 +28,7 @@ dsh-bash          ← dsh-brand                       (abstract executor seam; b
 dsh-session       ← dsh-llm, dsh-brand
 dsh-system-prompt ← dsh-llm
 dsh-agent         ← dsh-llm, dsh-session, dsh-brand
+dsh-compact       ← dsh-session, dsh-llm                (abstract compaction seam; backend + tool deferred)
 dsh-tools         ← dsh-llm, dsh-system-prompt, dsh-agent
 dsh-bash-local    ← dsh-bash                       (BashExecutor impl)
 dsh-tool-bash     ← dsh-bash, dsh-tools            (bash tool schemas)
@@ -58,6 +60,7 @@ The rule: **extension** plugins depend on interfaces, never on the concrete loop
 | `bash/` | `bash` | Abstract bash executor seam (interface + vocabulary) | `ctx.bash` |
 | `bash-local/` | `bash` | Local-subprocess `BashExecutor` implementation | (registers `ctx.bash`) |
 | `tool-bash/` | `bash` | Model-facing `bash`/`bash_output`/`bash_kill` tool schemas | (registers on `ctx.tools`) |
+| `compact/` | `compact` | Abstract compaction seam + `compact/*` events + `CompactionResult` | `ctx.compact` |
 | `llm-deepseek/` | `llm` | DeepSeek API adapter (hand-rolled fetch/SSE) | (registers on `ctx.llm`) |
 | `llm-pi-ai/` | `llm` | DeepSeek adapter via `@earendil-works/pi-ai` (design twin) | (registers on `ctx.llm`) |
 | `session-persistence/` | `session-persistence` | Persistence seam + write coordinator | `ctx.sessionPersistence` |
diff --git a/packages/compact/README.md b/packages/compact/README.md
new file mode 100644
index 0000000000..0d63b3b8cd
--- /dev/null
+++ b/packages/compact/README.md
@@ -0,0 +1,11 @@
+# compact/ — compaction capability family
+
+A three-package capability seam (see [capability seams](../../docs/rfc/implemented/architecture/2026-06-13-capability-seams.md)): an abstract compaction interface, a backend that summarizes, and the model-facing tool that consumes it. Only the interface tier exists today; the backend and consumer are deferred. All **product** packages.
+
+| Package | Role | ctx key |
+|---|---|---|
+| `compact/` | Abstract compaction seam (interface + `compact/*` events + `CompactionResult`) | `ctx.compact` |
+| `compact-basic/` (deferred) | A backend: char/4 estimation + token-budget retention + `llm.stream()` summarization | (registers `ctx.compact`) |
+| `tool-compact/` (deferred) | Model-facing `/compact` tool over `ctx.compact` | (registers on `ctx.tools`) |
+
+The interface lives at `compact/compact/`. Unlike the bash seam, it depends on `dsh-session` and `dsh-llm` — its verbs are defined over a `Session` and its output is the `ContentBlock` vocabulary, so the contract cannot be expressed without naming them. That deviation from the "interface depends only on cordis" guidance is intentional and recorded in the [compaction capability-seam RFC](../../docs/rfc/proposed/feature/2026-06-18-compaction-capability-seam.md). A tokenizer- or template-based backend would replace `compact-basic` without touching the interface or the tool.