fix(server): one failing RPC handler no longer ends the client's other requests

[juliusmarminge]

A defect (die) in any WebSocket RPC handler was sent as a socket-level Defect frame. When the client gets one it ends every pending request on that socket: the shell, thread and terminal subscriptions all fail at once for one bug in one handler. No ErrorReporter was installed either, so the defect never reached the server log.

Fix

• RpcServer.make(ServerWsRpcGroup, …) now passes disableFatalDefects: true. A defect fails only the request that raised it, as an ordinary Exit with a Die cause.
• New observability/DefectReporter.ts: an ErrorReporter that logs dies. It is provided to the WebSocket RPC handlers, whose request fibers RpcServer reports from, so handler defects go through the server's normal loggers (console and OTLP). It is not installed globally: McpServer already logs tool failures before reporting them, and a global reporter would log every MCP tool defect twice.
• packages/effect-acp had the same problem in two places:
  • Core methods go through RpcServer. A handler defect went out as a JSON-RPC error with id -32603 instead of the request's id, so the agent's request (a permission prompt, say) never got an answer and its turn hung. The client and agent RPC servers now pass disableFatalDefects: true.
  • Extension requests and notifications are handled in protocol.ts, outside RpcServer. There, matchEffect and Effect.ignore only caught typed failures, so a dying handler ended the stdin reader: that request and every later one on the connection went unanswered, and termination never fired. A dying extension handler now answers its own request with an internal error. Notification handlers (the protocol layer, the client's per-handler loop and the agent's cancel handlers) drop typed failures as before and log defects at Error, so one dying handler no longer stops the others or the reader.
  • A defect in a core or extension request handler is logged at Error before the agent gets its error; before, it reached no log.
  • A defect or an interrupt that ends the reader outside any handler, such as a session-update normalizer that throws, now terminates the connection, so pending requests fail instead of hanging.
  • The ACP RPC servers no longer inherit an ErrorReporter from the fiber that builds them. Built inside a WebSocket request (the ACP registry listing does this), they picked up DefectReporter and logged each handler defect twice.

Does the client need the Defect frame to reconnect?

No. packages/client-runtime/src/rpc/session.ts reconnects through ConnectionHooks.onDisconnect (the socket closing) and through the serverConfig subscription ending (session.closed). A Defect frame does neither. It only failed whatever was pending, and the config subscription failing was what made the supervisor reconnect. With this change, a defect in the config handler itself still fails that subscription with a Die, so session.closed and the reconnect still happen. Defects in other handlers no longer take the config subscription down with them. Durable subscriptions in rpc/client.ts already treat a Die exit on their own stream as a reported defect (onDefect), so thread state still shows "Could not synchronize the thread." for a defect in that subscription.

Protocol-level defects that are not tied to a request (invalid request ids, unknown message tags, unencodable frames) still send a socket-level Defect frame, and so does a handler or middleware that throws synchronously while building its effect. None of the WebSocket handlers do. disableFatalDefects and DefectReporter only cover defects inside the handler's effect. A response that fails to encode is sent to the client as a per-request Die but is not reported, as before.

Verification

• vp test run src/observability src/ws.test.ts src/terminal/OutputProtocol.test.ts in apps/server: 7 files, 31 tests pass.
  • The new RPC test pairs a real RpcClient with RpcServer using the same options as ws.ts and the reporter provided the same way. One handler dies while a sibling stream subscription on the same client keeps receiving, the dying call gets its own Die exit, and the defect is logged once.
  • With disableFatalDefects: false it fails (expected 'subscription ended' to equal 2): the client ends the sibling subscription. Without the reporter provided to the handlers, it fails on the missing log.
• vp test run src in packages/effect-acp: 6 files, 73 tests pass.
  • The client test lets a core handler, an extension request handler, an extension notification handler and a session/update handler die in turn on one connection. Both requests get their own error, a second session handler still runs, a later request is still answered, and each defect is logged once at Error. Reverting any one change fails it: disableFatalDefects (id -32603), the extension request or notification guard (later request never answered), the client's handler loop, the Error log level, or the request-handler log.
  • A protocol test lets the session-update normalizer throw and checks that the connection terminates with a transport error.
  • Waits in these tests are bounded, so a reverted guard fails within 2 s instead of at the test timeout.
  • The agent test checks that a dying initialize handler answers its own id with a Die cause.
• vp exec tsc --noEmit -p . in apps/server and packages/effect-acp: no errors or warnings.

Model/harness: Claude Opus 5.5 (1M context) via Claude Code in T3 Code.

🤖 Generated with Claude Code

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[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — The PR changes existing WebSocket and ACP production error-propagation defaults so handler defects become request-local and are logged, with effects spanning multiple runtime paths. The behavior is well tested, but the default change and cross-component blast radius warrant human review.

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[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Codex	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Codex	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Codex	Live turn WebSocket decoded	20.9 KiB	20.9 KiB	0 B (0.0%)	29.3 KiB	✅
Codex	Live turn messages	2	2	0 (0.0%)	8	✅
Claude	Total thread wire	5.0 KiB	5.0 KiB	0 B (0.0%)	6.8 KiB	✅
Claude	Thread snapshot wire	3.8 KiB	3.8 KiB	0 B (0.0%)	4.9 KiB	✅
Claude	Live turn WebSocket wire	1.2 KiB	1.2 KiB	0 B (0.0%)	2.0 KiB	✅
Claude	Live turn WebSocket decoded	21.2 KiB	21.2 KiB	0 B (0.0%)	29.3 KiB	✅
Claude	Live turn messages	2	2	0 (0.0%)	8	✅

Baseline: 9503155 · PR result: 6d36ef7 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 108.5 KiB
• Claude decoded thread snapshot: 108.8 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

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📝 Walkthrough

Walkthrough

The change adds logging for eligible RPC handler defects. WebSocket and ACP RPC servers disable fatal-defect handling. ACP notification failures are logged and ignored, while extension handler defects produce internal-error responses. Tests cover error responses, defect logging, and continued processing.

Changes

RPC defect handling

Layer / File(s)	Summary
Report defects in WebSocket RPC apps/server/src/observability/DefectReporter.ts, apps/server/src/observability/DefectReporter.test.ts, apps/server/src/ws.ts	The reporter logs non-ignored defects and skips typed failures and interrupts. WebSocket RPC disables fatal-defect handling and provides the reporter. Tests check defect logging and continued subscription delivery.
Handle ACP notification and extension failures packages/effect-acp/src/client.ts, packages/effect-acp/src/protocol.ts	Notification handler failures are logged and ignored. Extension handler causes with typed errors are converted to extension errors; other causes produce internal-error responses with method, operation, and cause details.
Configure ACP RPC defect responses packages/effect-acp/src/agent.ts, packages/effect-acp/src/agent.test.ts, packages/effect-acp/src/client.ts, packages/effect-acp/src/client.test.ts	The agent and compatibility RPC servers disable fatal-defect handling. Tests check defect responses, request IDs, and continued processing after handler defects.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: t3dotgg

Merge Risk: 🔵 Low · up to 20fdd

An extension request with both a typed failure and a handler defect can report only the typed failure. This is a bounded issue to fix or explicitly accept before merging.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 9…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly describes the primary change: a failing RPC handler no longer ends other requests on the client.
Description check	✅ Passed	The description explains the problem, the changes across server and ACP handlers, and focused verification results. It does not include the template’s Scope and approval information; add a link to the…

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Review comments at @apps/server/src/observability/DefectReporter.ts:
- Line 15: Remove the export from the make function in DefectReporter.ts; keep
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[juliusmarminge]

Server log before and after

What happens when a WS RPC handler dies (here serverGetConfig throws a TypeError) while another subscription is open on the same socket.

Setup:

• The real RpcServer/RpcClient pairing with the RPC server options from ws.ts. The handlers get the same DefectReporter.layer that ws.ts provides.
• The server's console logger (Logger.consolePretty(), as in serverLogger.ts).
• main runs { disableTracing: true } with no reporter. This PR runs WS_RPC_SERVER_OPTIONS plus DefectReporter.layer.

Before (main): the server logs nothing. The defect goes back to the client as a socket-level Defect, which also ends the other subscription.

(server log: empty)

# client: serverGetConfig ended with Cannot read properties of undefined (reading 'providers');
#         subscribeServerLifecycle (same socket) ended with Cannot read properties of undefined (reading 'providers')

After (this PR): the server logs the defect with its stack. Only the failing request fails, and the other subscription stays open.

[23:51:42.193] ERROR (#7): Unhandled defect
  Error: Cannot read properties of undefined (reading 'providers')
      at Object.serverGetConfig [as handler] (defect-log.probe.ts:25:37)
      at handleRequest (node_modules/.pnpm/effect@4.0.1_patch_hash=a33c…/node_modules/effect/dist/rpc/RpcServer.js:171:26)
      at RpcClient.serverGetConfig

# client: serverGetConfig ended with Cannot read properties of undefined (reading 'providers');
#         subscribeServerLifecycle (same socket) still open

Typed failures (declared RPC errors) and interrupts are not logged. Only dies are, as covered in DefectReporter.test.ts. When OTLP logs are configured, the same record is also exported as an Error log, stamped with the request's trace and span.