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fix(sdk): answer a malformed request with Invalid params, and log no error for a client's mistake - #966
Three issues found in the production logs of the docs MCP server at mcp.frontmcp.dev, a Cloudflare Worker on 1.10.11. In each case the client made a mistake, and the server either answered it wrongly or logged an error.
A malformed 2026-07-28 request reached its handler unchecked.dispatch20260728 calls the handler without checking the request against the handler's requestSchema. A tools/call with no name reached the tool flow, whose input parse threw a ZodError. The error handler logged ERROR Internal error: …, and the client got a successfultools/call result whose text was the raw ZodError and its stack. The dispatcher now checks the request against the handler's schema right after finding the handler. It answers -32602 with Invalid params: params.name: Invalid input: expected string, received undefined, before anything runs. It also checks inputResponses, a 2026-07-28 param no upstream schema covers, as an object of objects. Before, a string or a non-object answer went into the MRTR exchange as it was, and the tool ran.
finalize failed after a stage had already failed. When prompts/get or resources/read failed before producing output (an unknown prompt or resource, or a missing required argument), the runner still ran finalize. It found nothing in state, logged finalize: prompt not found in state (or resource …, or prompt output not found in state), and threw. The runner then logged [FrontMCP] Finalize stage error (suppressed to preserve original error). The client already had the right -32602. Both finalize stages now return when the runner recorded flowError, as execute-job's does. The runner rethrows the original error after finalize whenever it sets flowError, so skipping finalize there can't drop a response.
Every legacy initialize on a Worker warned twice. A server built for one request has no session registered with the notification service, so setClientCapabilities and setClientInfo warned Cannot set … for unregistered session and stored nothing. The initialize handler now calls them only when getRegisteredServer(sessionId) finds the session. Behavior is otherwise unchanged: the session store still gets the capabilities, and the platform is still decided as before.
Not changed here: on the legacy (pre-2026-07-28) path, a malformed request is answered -32603 with the raw Zod issues. That comes from the MCP SDK (1.32.1), not FrontMCP. Server.setRequestHandler wraps tools/call to answer InvalidParams, but registers the wrapper through Protocol.setRequestHandler, which parses the request first and throws a bare ZodError.
User-facing change
What changed: On MCP 2026-07-28, a request whose params don't fit its method gets JSON-RPC -32602 with Invalid params: <field>: <problem>.
Before → after: A tools/call with no name got a successful result whose text was the raw ZodError, with an internal error logged → -32602Invalid params: params.name: Invalid input: expected string, received undefined, with nothing logged. Malformed inputResponses gets -32602 naming params.inputResponses, and the tool doesn't run. Unknown prompts, unknown resources and missing prompt arguments get the same answers as before, without the two logged finalize errors. A legacy initialize on a Worker no longer logs the two unregistered session warnings.
frontmcp.dev pages affected (if known): Error reference pages listing what a malformed request is answered with, if they cover it.
Skills catalog entries updated: None. The catalog doesn't describe a malformed request's answer.
Tests
New libs/sdk/src/transport/__tests__/web-fetch-handler.request-errors.spec.ts (8 tests). It drives a real web-fetch server with logging at warn and asserts on the answers and on console output. Before the fix, the five original tests fail with the same log lines the production Worker logged, and the two malformed-inputResponses tests fail because the tool runs. A third test checks that well-formed inputResponses still runs the tool. After the fix, all eight pass.
libs/sdk/src/{transport,task,elicitation} specs, after the inputResponses check: 1,281 tests passing, MRTR included.
initialize-request.handler.spec.ts: the notification mock now has getRegisteredServer, and a new test checks that a session with no registered server stores no client details while the session store still gets the capabilities.
libs/sdk/src/{transport,prompt,resource,tool,notification,scope,flows} specs: 1,994 of 1,995 pass. The one failure was a 5-second timeout in scope-ha-redis-client.spec.ts under batch load; it passes on its own and doesn't touch this change.
nx build sdk, tsc over the spec config (0 errors), ESLint (no new warnings), Prettier and scripts/fix-unused-imports.mjs are clean.
Summary by CodeRabbit
Bug Fixes
Invalid requests now receive an “Invalid params” response identifying the request issues.
Malformed input responses are rejected before the requested tool runs.
Failed prompt and resource flows no longer continue final processing or return output.
Initialization requests for unregistered sessions no longer store client details in the notification service; session capability persistence remains unchanged.
Legacy initialization requests using the supported protocol version continue to receive a successful response without an unregistered-session warning.
…error for a client's mistake
A 2026-07-28 request went to its handler without being checked against
the handler's request schema. A tools/call with no name reached the tool
flow, whose input parse threw a ZodError; the error handler logged it as
an internal error and answered with a successful tools/call result
carrying the raw ZodError text. The dispatcher now checks the request
against the handler's schema first and answers -32602 Invalid params,
naming each field it gets wrong, before anything runs.
When prompts/get or resources/read failed before producing output (an
unknown prompt or resource, a missing required argument), the flow runner
still ran the finalize stage. It found no prompt, resource or output in
state, logged an error and threw a second one, which the runner logged as
a suppressed finalize error. The client already had the right answer.
Both finalize stages now return when the runner recorded flowError, as
the job flow's finalize does.
An initialize on a server built for one request, as on a Worker, has no
session registered with the notification service, so storing the
client's capabilities and info warned twice and stored nothing. The
initialize handler now stores them only when the session has a
registered server.
Included review availability: This review used your included allowance. 4 included reviews remain after this review. Your included PR review attempts over the past 7 days set your current allowance at 5 reviews per hour.
Walkthrough
The SDK now skips prompt and resource output handling when a flow error is set. The dispatcher validates request bodies against handler schemas and the optional inputResponses field. The initialize handler limits notification-service writes to sessions with a registered server.
The dispatcher validates request bodies against the matched handler schema and the optional inputResponses field. Invalid requests return JSON-RPC error -32602. Tests cover invalid and valid parameters, unknown prompts and resources, and legacy initialization.
The handler writes client capabilities and client information to the notification service only when a registered server exists. The test verifies that capabilities are still persisted to the session store when no registered server exists.
Valid requests remain accepted, and malformed inputResponses is rejected before the tool runs. No outstanding issue identified here prevents merging.
Pre-merge checks: 5
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Title check
The title clearly identifies the main change: malformed requests now receive Invalid params responses and client mistakes do not create error logs.
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The description follows the required sections and explains the changes, user-facing behavior, affected documentation, tests, and validation results. It also reports the one unrelated timeout instead o…
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The description follows the required sections and explains the changes, user-facing behavior, affected documentation, tests, and validation results. It also reports the one unrelated timeout instead of omitting it.
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Docstring coverage is 100.00% which is sufficient. The required threshold is 65.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 2 functions across 6 files.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @libs/sdk/src/transport/mcp-20260728/dispatcher.ts:
- Line 371: In the dispatcher flow around describeInvalidParams, add
2026-specific validation of body.inputResponses before constructing the
MrtrExchange, rejecting malformed values with -32602 so the tools/call handler
does not run. Preserve the existing requestState handling that restarts the
exchange for malformed, expired, or invalid-signature state.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
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inputResponses arrives only on 2026-07-28 requests, so the upstream
request schemas the dispatcher now checks don't cover it. A string, or
an answer that isn't an object, went into the MRTR exchange as it was,
and the tool ran. The dispatcher now also checks inputResponses as an
object of objects, and answers -32602 naming params.inputResponses.
…, and log no error for a client's mistake (#967)
Cherry-picked from #966 (merged to release/1.10.x)
Original commit: cd91cc3
Co-authored-by: agentfront[bot] <agentfront[bot]@users.noreply.github.com>
Co-authored-by: frontegg-david <69419539+frontegg-david@users.noreply.github.com>
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What and why
Three issues found in the production logs of the docs MCP server at mcp.frontmcp.dev, a Cloudflare Worker on 1.10.11. In each case the client made a mistake, and the server either answered it wrongly or logged an error.
dispatch20260728calls the handler without checking the request against the handler'srequestSchema. Atools/callwith nonamereached the tool flow, whose input parse threw aZodError. The error handler loggedERROR Internal error: …, and the client got a successfultools/callresult whose text was the rawZodErrorand its stack. The dispatcher now checks the request against the handler's schema right after finding the handler. It answers-32602withInvalid params: params.name: Invalid input: expected string, received undefined, before anything runs. It also checksinputResponses, a 2026-07-28 param no upstream schema covers, as an object of objects. Before, a string or a non-object answer went into the MRTR exchange as it was, and the tool ran.finalizefailed after a stage had already failed. Whenprompts/getorresources/readfailed before producing output (an unknown prompt or resource, or a missing required argument), the runner still ranfinalize. It found nothing in state, loggedfinalize: prompt not found in state(orresource …, orprompt output not found in state), and threw. The runner then logged[FrontMCP] Finalize stage error (suppressed to preserve original error). The client already had the right-32602. Bothfinalizestages now return when the runner recordedflowError, asexecute-job's does. The runner rethrows the original error afterfinalizewhenever it setsflowError, so skippingfinalizethere can't drop a response.initializeon a Worker warned twice. A server built for one request has no session registered with the notification service, sosetClientCapabilitiesandsetClientInfowarnedCannot set … for unregistered sessionand stored nothing. Theinitializehandler now calls them only whengetRegisteredServer(sessionId)finds the session. Behavior is otherwise unchanged: the session store still gets the capabilities, and the platform is still decided as before.Not changed here: on the legacy (pre-2026-07-28) path, a malformed request is answered
-32603with the raw Zod issues. That comes from the MCP SDK (1.32.1), not FrontMCP.Server.setRequestHandlerwrapstools/callto answerInvalidParams, but registers the wrapper throughProtocol.setRequestHandler, which parses the request first and throws a bareZodError.User-facing change
-32602withInvalid params: <field>: <problem>.tools/callwith nonamegot a successful result whose text was the rawZodError, with an internal error logged →-32602Invalid params: params.name: Invalid input: expected string, received undefined, with nothing logged. MalformedinputResponsesgets-32602namingparams.inputResponses, and the tool doesn't run. Unknown prompts, unknown resources and missing prompt arguments get the same answers as before, without the two logged finalize errors. A legacyinitializeon a Worker no longer logs the twounregistered sessionwarnings.Tests
libs/sdk/src/transport/__tests__/web-fetch-handler.request-errors.spec.ts(8 tests). It drives a real web-fetch server with logging atwarnand asserts on the answers and onconsoleoutput. Before the fix, the five original tests fail with the same log lines the production Worker logged, and the two malformed-inputResponsestests fail because the tool runs. A third test checks that well-formedinputResponsesstill runs the tool. After the fix, all eight pass.libs/sdk/src/{transport,task,elicitation}specs, after theinputResponsescheck: 1,281 tests passing, MRTR included.initialize-request.handler.spec.ts: the notification mock now hasgetRegisteredServer, and a new test checks that a session with no registered server stores no client details while the session store still gets the capabilities.libs/sdk/src/{transport,prompt,resource,tool,notification,scope,flows}specs: 1,994 of 1,995 pass. The one failure was a 5-second timeout inscope-ha-redis-client.spec.tsunder batch load; it passes on its own and doesn't touch this change.nx build sdk,tscover the spec config (0 errors), ESLint (no new warnings), Prettier andscripts/fix-unused-imports.mjsare clean.Summary by CodeRabbit