diff --git a/docs/config-catalog.md b/docs/config-catalog.md
index d437649d9b..b28634ccae 100644
--- a/docs/config-catalog.md
+++ b/docs/config-catalog.md
@@ -688,7 +688,7 @@ export interface ReplayModelConfig {
 }
 ```
 
-Source: [`packages/support/llm-replay/src/index.ts:459`](../packages/support/llm-replay/src/index.ts)
+Source: [`packages/support/llm-replay/src/index.ts:496`](../packages/support/llm-replay/src/index.ts)
 
 ## `@deepseek-ai/dsh-llm-retry`
 
diff --git a/packages/support/llm-replay/README.md b/packages/support/llm-replay/README.md
index a8d811f35b..50c4976b8d 100644
--- a/packages/support/llm-replay/README.md
+++ b/packages/support/llm-replay/README.md
@@ -8,7 +8,7 @@ Its consumers are the ACP snapshot harness in `examples/acp-agent` and the `stre
 
 The fixture IS the persisted session log (`<scenario>/session.jsonl`). Its `assistant/chunk` events carry every `StreamChunk`, so grouping them by `(turn, step)` reconstructs each `stream()` call's chunk sequence (one model call per loop step). Recording is therefore "run the real agent once and harvest the `.jsonl`", done by the snapshot harness — this plugin does not record. A fixture may carry its `request/header` content tokenized to `{{system}}`/`{{tools}}` (the harness pins that content in one scenario and scrubs the rest); replay is indifferent — derivation reads only `assistant/chunk` events and the line-0 session header.
 
-Two failure modes are not reconstructable from `assistant/chunk` alone — a pure throw before any chunk (e.g. an HTTP 401, where the log holds only a `turn/end {error}` and no chunks) and a cancel/hang (timing, not chunk content). A scenario that needs those supplies an optional sidecar (`<scenario>/replay.override.json`: a `ReplayEntry[]`) that REPLACES the derived script. A `hang` entry may name `readyFile`; replay writes that empty marker after its prefix chunks reach the loop and before it waits for cancellation, so an external driver can cancel deterministically without observing a presentation update.
+Two failure modes are not reconstructable from `assistant/chunk` alone — a pure throw before any chunk (e.g. an HTTP 401, where the log holds only a `turn/end {error}` and no chunks) and a cancel/hang (timing, not chunk content). A scenario that needs those supplies an optional sidecar (`<scenario>/replay.override.json`) that either REPLACES the derived script (a bare `ReplayEntry[]`) or AUGMENTS it (`{ patches: [{ at, entry }] }`: keep every JSONL-derived call, swap only the named 0-based call indexes; `at` equal to the derived length appends — the slot for the retry attempt that follows an injected transient throw). A `hang` entry may name `readyFile`; replay writes that empty marker after its prefix chunks reach the loop and before it waits for cancellation, so an external driver can cancel deterministically without observing a presentation update.
 
 ## Nested agents: per-session keying
 
diff --git a/packages/support/llm-replay/src/index.ts b/packages/support/llm-replay/src/index.ts
index f9637536f8..e54eabc4b6 100644
--- a/packages/support/llm-replay/src/index.ts
+++ b/packages/support/llm-replay/src/index.ts
@@ -200,26 +200,63 @@ export function deriveReplayScript(events: SessionEvent[]): ReplayEntry[] {
 }
 
 /**
- * Build the replay script for the PRIMARY session: the sidecar override if
- * present, otherwise the script derived from the recorded session JSONL.
- * Fail-loud if the JSONL fixture is missing (the scenario was never recorded) —
- * never silently returns an empty script, so a coverage hole can't masquerade
- * as a passing replay.
+ * One positional patch in an augmentation sidecar: replaces the derived
+ * entry at call index `at` (0-based) with `entry`, or appends when `at`
+ * equals the derived length (an extra recorded-after-the-fact call, e.g. the
+ * retry attempt following an injected transient throw).
+ */
+export interface ReplayOverridePatch {
+  /** 0-based call index into the derived script; == length appends. */
+  at: number
+  /** The replacement (or appended) entry at that call position. */
+  entry: ReplayEntry
+}
+
+/**
+ * Override sidecar document: either the legacy whole-script replacement (a
+ * bare `ReplayEntry[]`) or the augmentation form `{ patches }`, which keeps
+ * the JSONL-derived script and swaps only the named call indexes — the shape
+ * for "turn N errors, everything else replays as recorded".
+ */
+export type ReplayOverrideDoc = ReplayEntry[] | { patches: ReplayOverridePatch[] }
+
+/**
+ * Load the PRIMARY session's replay script: the sidecar override when present
+ * (whole-script replacement or `{ patches }` augmentation over the derived
+ * script), else the script derived from the session JSONL (fail-loud when the
+ * fixture is missing).
  * @param config - the fixture paths; only `file` and `overrideFile` are consulted.
- * @returns the primary session's replay entries.
+ * @returns the resolved primary-session script.
  */
 export function loadReplayScript(config: ReplayConfig): ReplayEntry[] {
   if (config.overrideFile !== undefined && existsSync(config.overrideFile)) {
     const parsed: unknown = JSON.parse(readFileSync(config.overrideFile, 'utf8'))
-    if (!Array.isArray(parsed)) {
-      throw new Error(`llm-replay: override is not a JSON array: ${config.overrideFile}`)
+    if (Array.isArray(parsed)) return parsed as ReplayEntry[]
+    const doc = parsed as { patches?: unknown }
+    if (typeof parsed !== 'object' || parsed === null || !Array.isArray(doc.patches)) {
+      throw new Error(`llm-replay: override must be a ReplayEntry[] or { patches: [...] }: ${config.overrideFile}`)
     }
-    return parsed as ReplayEntry[]
+    const script = deriveScriptFromFile(config.file)
+    for (const patch of doc.patches as ReplayOverridePatch[]) {
+      if (!Number.isInteger(patch.at) || patch.at < 0 || patch.at > script.length) {
+        throw new Error(
+          `llm-replay: override patch index ${String(patch.at)} out of range `
+          + `(derived script has ${script.length} call(s); == length appends): ${config.overrideFile}`,
+        )
+      }
+      script[patch.at] = patch.entry
+    }
+    return script
   }
-  if (!existsSync(config.file)) {
-    throw new Error(`llm-replay: fixture not found: ${config.file} — run \`pnpm run test:snapshot:record\` first`)
+  return deriveScriptFromFile(config.file)
+}
+
+/** Derive the primary script from the session JSONL, failing loud on a missing fixture. */
+function deriveScriptFromFile(file: string): ReplayEntry[] {
+  if (!existsSync(file)) {
+    throw new Error(`llm-replay: fixture not found: ${file} — run \`pnpm run test:snapshot:record\` first`)
   }
-  return deriveReplayScript(parseSessionLog(readFileSync(config.file, 'utf8')))
+  return deriveReplayScript(parseSessionLog(readFileSync(file, 'utf8')))
 }
 
 /**
diff --git a/packages/support/llm-replay/tests/llm-replay.spec.ts b/packages/support/llm-replay/tests/llm-replay.spec.ts
index 584a87abf6..1bd8d47405 100644
--- a/packages/support/llm-replay/tests/llm-replay.spec.ts
+++ b/packages/support/llm-replay/tests/llm-replay.spec.ts
@@ -207,7 +207,52 @@ describe('loadReplayScript', () => {
     writeFileSync(file, sessionJsonl([]), 'utf8')
     const overrideFile = join(dir, 'replay.override.json')
     writeFileSync(overrideFile, '{"not":"array"}', 'utf8')
-    expect(() => loadReplayScript({ file, overrideFile })).toThrow(/not a JSON array/)
+    expect(() => loadReplayScript({ file, overrideFile })).toThrow(/ReplayEntry\[\] or \{ patches/)
+  })
+
+  it('patches form: swaps the named call index and keeps derived siblings', () => {
+    const callB: StreamChunk[] = [
+      { type: 'block-start', index: 0, blockType: 'text' },
+      { type: 'text-delta', index: 0, text: 'two' },
+      { type: 'finish', reason: { kind: 'stop' } },
+    ]
+    let seq = 1
+    writeFileSync(file, sessionJsonl([
+      ...TEXT_CHUNKS.map(c => chunkEvent(seq++, 1, 1, c)),
+      ...callB.map(c => chunkEvent(seq++, 1, 2, c)),
+    ]), 'utf8')
+    const overrideFile = join(dir, 'replay.override.json')
+    writeFileSync(overrideFile, JSON.stringify({
+      patches: [{ at: 0, entry: { kind: 'throw', chunks: [], message: 'transient', code: 'SERVER' } }],
+    }), 'utf8')
+    expect(loadReplayScript({ file, overrideFile })).toEqual([
+      { kind: 'throw', chunks: [], message: 'transient', code: 'SERVER' },
+      { kind: 'chunks', chunks: callB },
+    ])
+  })
+
+  it('patches form: at == derived length appends (the retry-attempt slot)', () => {
+    writeFileSync(file, sessionJsonl(TEXT_CHUNKS.map((c, i) => chunkEvent(i + 1, 1, 1, c))), 'utf8')
+    const overrideFile = join(dir, 'replay.override.json')
+    writeFileSync(overrideFile, JSON.stringify({
+      patches: [
+        { at: 0, entry: { kind: 'throw', chunks: [], message: '429', code: 'RATE_LIMIT' } },
+        { at: 1, entry: { kind: 'chunks', chunks: TEXT_CHUNKS } },
+      ],
+    }), 'utf8')
+    expect(loadReplayScript({ file, overrideFile })).toEqual([
+      { kind: 'throw', chunks: [], message: '429', code: 'RATE_LIMIT' },
+      { kind: 'chunks', chunks: TEXT_CHUNKS },
+    ])
+  })
+
+  it('patches form: an out-of-range index fails loud with the derived length', () => {
+    writeFileSync(file, sessionJsonl(TEXT_CHUNKS.map((c, i) => chunkEvent(i + 1, 1, 1, c))), 'utf8')
+    const overrideFile = join(dir, 'replay.override.json')
+    for (const at of [2, -1, 1.5]) {
+      writeFileSync(overrideFile, JSON.stringify({ patches: [{ at, entry: { kind: 'hang' } }] }), 'utf8')
+      expect(() => loadReplayScript({ file, overrideFile })).toThrow(/patch index .* out of range/)
+    }
   })
 })