perf(coding-agent): cap cell source attached to kernel host requests (#2475)

.pylon/upstream-review.md

@@ -446,7 +446,7 @@ after exact owned cleanup and supervisor exit. Pylon delivery tracked in [pylon#
 | #2533, #2555, #2560 (`1e2cc2278`, `b11fd5e2d`, `346a463da`) | Claude Opus 5.5 adaptive thinking | **Adopt** | Configures always-on adaptive thinking and test fixture surfaces for Claude Opus 5.5. |
 | #2645 (`703cc4547`) | Claude Code client version bump (2.1.281) & ban risk notice | **Adopt** | Claims client version 2.1.281 for Anthropic subscription OAuth requests to pass backend model gating, accompanied by user-facing ban-risk warning. |
 | #2507 (`36b912fea`) | Remote catalog fetch from `prime-agent-catalog` | **Hold / Audit** | Introduces runtime network dependency for model metadata. Held pending evaluation of offline cache fallbacks and deterministic air-gapped guarantees. |
-| #2475 (`561401b27`) | Truncate cell source to 2KB on `host_request` | **Hold / Audit** | May break downstream provenance reconstruction and context hash verification when cells exceed 2KB. Held for Pylon trace analysis. |
+| #2475 (`561401b27`) | Truncate cell source to 2KB on `host_request` | **Adopt** | Caps spawning cell source attached to host requests at 2KB with truncation marker. Verified that Pylon does not consume `cellSourceCode` or depend on >2KB cell source for provenance. |
 | #2382 (`d73349d50`) | Raw `child.kill("SIGKILL")` on worker bridge EPIPE | **Reject** | Violates Pylon syscall safety invariant; raw signals to unverified PIDs risk killing recycled processes during rapid worker crashes. Must rely on supervised process exit receipts. |
 | #2471 (`8ee46be35`), #2478 (`02e5babb4`) | Live mutation of CPython `__closure__` and `__globals__` | **Reject** | Unsafe runtime bytecode/closure mutation in REPL state restore risks memory leaks and CPython interpreter `SIGSEGV` crashes. |
 | #2388 (`1c1ad1e48`) | Synchronous session name reclamation on delete receipt | **Reject** | Deleting session name before child process unwinding completes creates race conditions with pending detached worker handles. |

packages/coding-agent/.changes/cap-spawn-cell-source.md

@@ -0,0 +1 @@
+- Capped the spawning cell source attached to kernel host requests at 2KB with a truncation marker, so oversized cells stop re-shipping their full source on every spawn/progress-note/collect round trip and in each child's persisted spawnCode.

packages/coding-agent/src/core/kernel/repl-manager.ts

@@ -74,6 +74,11 @@ const MAX_BACKGROUND_OUTPUT_CHARS = 64 * 1024;
 // MAX_ATTACHMENT_DATA_CHARS; a line that cannot complete within this ceiling is
 // corruption the protocol repair owns, not output worth buffering until OOM.
 const MAX_PROTOCOL_LINE_CHARS = 32 * 1024 * 1024;
+// The spawning cell's source rides every host-request round trip and persists per child
+// as runtimeMetadata.spawnCode, so cap it once at this boundary.
+// Keep below SPAWN_CODE_MAX_CHARS in daemon-session-list.ts so its 4000-char display slice stays a no-op.
+const MAX_CELL_SOURCE_CHARS = 2 * 1024;
+const CELL_SOURCE_TRUNCATION_MARKER = ` [... cell source truncated at ${MAX_CELL_SOURCE_CHARS} chars ...]`;
 
 const MAX_KERNEL_STDERR_CHARS = 8 * 1024;
 const MAX_KERNEL_STDERR_LOG_BYTES = 5 * 1024 * 1024;
@@ -91,6 +96,11 @@ function writeFullySync(fd: number, data: Buffer): void {
 	}
 }
 
+function capCellSourceCode(code: string | undefined): string | undefined {
+	if (code === undefined || code.length <= MAX_CELL_SOURCE_CHARS) return code;
+	return `${code.slice(0, MAX_CELL_SOURCE_CHARS)}${CELL_SOURCE_TRUNCATION_MARKER}`;
+}
+
 /** ExecuteResult plus the raw fields of the request's `done` event (state ops). */
 interface InternalExecuteResult extends ExecuteResult {
 	doneFields?: Record<string, unknown>;
@@ -1331,7 +1341,7 @@ export class ReplKernelManager {
 		// Tag the request with the cell that triggered it. A blocking call is still
 		// the in-flight execution; detached spawns (asyncio.create_task) fire after
 		// the scheduling cell goes idle, so fall back to that last cell's source.
-		const cellSourceCode = this.activeExecution?.code ?? this.lastCellCode;
+		const cellSourceCode = capCellSourceCode(this.activeExecution?.code ?? this.lastCellCode);
 		return handler({ ...data, cellSourceCode });
 	}
 

packages/coding-agent/test/repl-kernel-execute.test.ts

@@ -120,6 +120,11 @@ describeIf("ReplKernelManager execute (real runtime)", () => {
 		const unknown = await manager.execute("import rlm\nawait rlm.host_request('test.unknown')");
 		expect(unknown.status).toBe("error");
 		expect(unknown.error?.evalue).toContain('host request type "test.unknown" is not available');
+
+		const padded = `${"# pad\n".repeat(500)}import rlm\nreply = await rlm.host_request('test.echo', {'value': 9, 'cellSourceCode': 'FAKE'})\n[len(reply['cell']), reply['cell'].endswith(' [... cell source truncated at 2048 chars ...]'), reply['cell'] == 'FAKE']`;
+		const capped = await manager.execute(padded);
+		expect(capped.status).toBe("ok");
+		expect(capped.result).toBe("[2094, True, False]");
 	}, 30_000);
 
 	it("spawns through rlm.spawn over the unchanged rlm.run wire type, requires a child name, and refuses a direct rlm call", async () => {