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fix(mcp): replay permit_wallet_mismatch 403 with signed denial receipt and details (SEL-482) - #218
The first permit_wallet_mismatch refusal stored a signed denial receipt, but its permit validation details were not persisted. On replay, validate_replay_access rejected the foreign permit before the stored denial could be returned, producing only detail.error.
Fix
Store the denial details with the terminal idempotency response. Permit replay of that specific stored 403 when the signed denial receipt is bound to the requesting wallet, API key, permit, and tool. Rebuild the replayed ToolPermissionDenied with the stored receipt and details.
Test
Added gateway/tests/test_mcp_permit_denial_replay.py to assert the first and replayed 403 have identical response bytes, including the signed denial receipt and {"bound_to":"subject_wallet_id"} details. It also checks that a second API key on the same wallet cannot retrieve the stored receipt.
Relevant pytest: 96 passed.
Ruff 0.17.1 check and format check: passed across all discovered Python projects.
Commit hooks: passed, including mypy and secret checks.
PR #210 (hermes/sel-475-cross-tenant-replay) contains a separate strict xfail for this documented behavior. This PR does not edit #210; its marker should be removed when that test is integrated with this fix.
Fixes SEL-482
Note
Medium Risk
Changes governed MCP idempotency replay and permit replay-access checks on security-sensitive denial paths; behavior is narrowed to a specific stored 403 shape but incorrect matching could affect cross-tenant refusal semantics.
Overview
Fixes idempotent replay for governed MCP calls that were denied with permit_wallet_mismatch: retries now return the same 403 body as the first response, including the signed denial receipt and permit details (e.g. {"bound_to": "subject_wallet_id"}), on REST invoke and standard JSON-RPC /mcp.
Persistence: permit validation denials now write permit_validation.details into the terminal idempotency payload via response_extra_data on _finalize_governed_denial.
Replay path:_assert_governed_replay_access receives the stored replay and, for this specific stored 403, skips re-denying when the cached receipt matches the caller’s wallet, key, permit, and tool. _raise_replayed_error forwards stored details into ToolPermissionDenied.
Tests: removes the strict xfail on cross-tenant stored-refusal byte identity and adds test_mcp_permit_denial_replay.py (byte-identical replay, other API key cannot recover the full stored denial).
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Reviewing files that changed from the base of the PR and between c6e9ed3 and 5161a6b.
Files selected for processing: 1
gateway/tests/test_cross_tenant_refusal_replay.py
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Walkthrough
Permit-denial finalization stores validation details in the response. Replay-access checks receive the stored replay record and allow a stored 403 denial only when its receipt matches the requesting wallet, key, permit, and tool. Replayed 403 errors preserve dictionary-valued details. Integration tests check replay responses, receipt validity, restrictions for a different API key, wallet-ID non-disclosure, and the absence of tool effects.
The replay change preserves the signed denial receipt and details while restricting access to matching credentials.
Security architecture risk: Low · up to 5161a
The change narrowly permits callers to retrieve their own previously recorded denial without authorizing tool execution. Matching wallet, API key, permit, and tool checks limit exposure. Compatibility with older records and interrupted finalization remains less certain.
Retained concerns
No architecture-level concerns identified.
Security review details
Security Blast Radius
inferred — The changed authority is access to denial evidence, not permission to execute a foreign permit. For this exception, the independently accessible scope is constrained to a matching stored request and receipt bound to the current wallet, API key, permit, and tool. No broader tenant or service authority was established.
Trust Boundaries and Controls
observed — The replay gate first performs existing permit replay validation. Only permit_wallet_mismatch may use the exception, requiring stored status 403, the same error reason, and matching receipt identity and denied outcome. Other validation failures remain denied. The exception relies on server-owned persisted receipt fields and does not independently verify their signature at this gate.
Resilience and Maintainability Implications
observed — Receipt creation precedes a separate idempotency completion write. Completion retries only persistence, not the caller's action, and exhaustion does not invent a terminal result or release the key. These existing controls are unchanged; the reviewed regressions cover sequential replay rather than interruption or concurrent recovery.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
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Explanation
Resolution
Docstring Coverage
Docstring coverage is 42.86% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 3 files.
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
Check name
Status
Explanation
Title check
The title clearly and concisely describes the main change: replaying permit_wallet_mismatch 403 responses with the signed denial receipt and details.
Description check
The description directly explains the cause, fix, affected replay paths, security constraints, tests, and validation results.
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
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Actionable comments posted: 1
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Inline comments:
Review comments at @gateway/tests/test_mcp_permit_denial_replay.py:
- Around line 88-90: In the replay receipt assertions, also verify that key_id
matches tenant_b["key_id"], permit_id matches permit_a["permit_id"], and tool
matches counted_tool.name. Keep the existing wallet_id and outcome assertions.
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Reviewing files that changed from the base of the PR and between 1a92592 and 39528f4.
Files selected for processing: 2
gateway/app/routers/mcp.py
gateway/tests/test_mcp_permit_denial_replay.py
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permit_wallet_mismatch 403 replay is byte-identical: existing test asserts replay.content == first.content, details {"bound_to":"subject_wallet_id"}, receipt signature valid, other key gets bare 403, no effects. Local uncommitted supplemental test (receipt key_id/permit_id/tool, detail.details + receipt equality, content-type) passes.
Blocking: 1 unresolved CodeRabbit thread asking for exactly those receipt key_id/permit_id/tool asserts — add them to the test. Gap: replay over JSON-RPC /mcp untested.
Common: heads verified, required CI green, git merge-tree clean vs origin/main, no force-push (fast-forward from prior r17 heads), no fixture/test removals in diff vs main. Full gateway suites: only test_acp_checkout_end_to_end_latency_budget failed under 5-way parallel load; it passes serially (24/24 test_acp_bridge) on every head.
Head 2ac0ea4, clean merge, CI green, 0 open threads (GraphQL), fast-forward, no fixtures removed.
Receipt assertions (wallet, key_id, permit_id, tool, outcome, credits=0, signature) plus JSON-RPC /mcp replay test (-32003 with details identical on replay, receipt verifies, no charge).
DeepSeek never passed the old head (2ac0ea4), so this is a full review of main...b33221f, not just the delta.
The remerge-diff of b33221f (merge of main d8ade3e) is empty.
gateway/app/routers/mcp.py:1693: the permit_wallet_mismatch denial now stores details. Every other denial path is unchanged.
gateway/app/routers/mcp.py:4080-4097: the replay bypass is narrow. It needs all of these: the validation reason is permit_wallet_mismatch, the stored status is 403, the stored error equals the reason, and the stored signed receipt's wallet_id, key_id, permit_id and tool all match the caller, with outcome denied.
It only skips the raise. Control then goes to _raise_replayed_error, so the foreign permit can never execute anything.
Success replays and other keys are still refused. The test covers a second key on the same wallet.
gateway/app/routers/mcp.py:4283: on replay, details is passed only when it's a dict.
The SEL-475 strict xfail is removed, and that test now passes.
Verified locally:
test_mcp_permit_denial_replay.py and test_cross_tenant_refusal_replay.py: 5 passed.
With mcp.py reverted to main, both new tests fail.
CI is green (CodeRabbit still pending; it's not a required check).
This is a delta review from b33221f, which DeepSeek passed earlier (#218 (review)).
c6e9ed3 is a merge of main b4bbc39 (bringing in #226 and #228). Its remerge-diff is empty.
The PR's diff against main is unchanged: gateway/app/routers/mcp.py, test_cross_tenant_refusal_replay.py and test_mcp_permit_denial_replay.py (+204/-15).
The 6 files changed b33221f..c6e9ed3 come only from main.
CI is green (CodeRabbit still pending; it's not a required check).
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QA PASS — independent Codex review at head fac4e31. Verified wallet-scoped replay lookup, request and signed-receipt identity bindings, and no cross-wallet or second-key replay bypass. Focused gateway tests: 87 passed; Ruff check and format passed. Current-head CI result, License result, and gitleaks are green; all review threads are resolved and the branch is current with main. Muse identified a nonblocking negative-test coverage opportunity; no code bypass was found.
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Cause
The first
permit_wallet_mismatchrefusal stored a signed denial receipt, but its permit validation details were not persisted. On replay,validate_replay_accessrejected the foreign permit before the stored denial could be returned, producing onlydetail.error.Fix
Store the denial details with the terminal idempotency response. Permit replay of that specific stored 403 when the signed denial receipt is bound to the requesting wallet, API key, permit, and tool. Rebuild the replayed
ToolPermissionDeniedwith the stored receipt and details.Test
Added
gateway/tests/test_mcp_permit_denial_replay.pyto assert the first and replayed 403 have identical response bytes, including the signed denial receipt and{"bound_to":"subject_wallet_id"}details. It also checks that a second API key on the same wallet cannot retrieve the stored receipt.PR #210 (
hermes/sel-475-cross-tenant-replay) contains a separate strict xfail for this documented behavior. This PR does not edit #210; its marker should be removed when that test is integrated with this fix.Fixes SEL-482
Note
Medium Risk
Changes governed MCP idempotency replay and permit replay-access checks on security-sensitive denial paths; behavior is narrowed to a specific stored 403 shape but incorrect matching could affect cross-tenant refusal semantics.
Overview
Fixes idempotent replay for governed MCP calls that were denied with
permit_wallet_mismatch: retries now return the same 403 body as the first response, including the signed denial receipt and permit details (e.g.{"bound_to": "subject_wallet_id"}), on REST invoke and standard JSON-RPC/mcp.Persistence: permit validation denials now write
permit_validation.detailsinto the terminal idempotency payload viaresponse_extra_dataon_finalize_governed_denial.Replay path:
_assert_governed_replay_accessreceives the stored replay and, for this specific stored 403, skips re-denying when the cached receipt matches the caller’s wallet, key, permit, and tool._raise_replayed_errorforwards storeddetailsintoToolPermissionDenied.Tests: removes the strict
xfailon cross-tenant stored-refusal byte identity and addstest_mcp_permit_denial_replay.py(byte-identical replay, other API key cannot recover the full stored denial).Reviewed by Cursor Bugbot for commit 2ac0ea4. Bugbot is set up for automated code reviews on this repo. Configure here.