perf(mcp): cap mem_context output budget (#1039) - #1074
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Route the mem_context tool through FormatContextWithOptions instead of the unbounded legacy FormatContext path. The MCP transport now always renders a bounded context: 16 KiB total budget by default (the Gentleman-Programming#1012 Claude hook budget), 20 pinned rows, and optional max_bytes/compact tool arguments mirroring the /context query params. max_bytes is clamped to the same 64 KiB ceiling the HTTP endpoint enforces, and non-positive or mistyped input falls back to the default so the MCP path can never resolve to unbounded output. The tool contract fixture gains the two new declared properties.
…ogramming#1039) int(float64 in (0,1)) truncates to 0, which ContextOptions treats as the unbounded legacy budget, so a max_bytes of 0.5 silently re-opened the unbounded path the perf change closes. Sub-integer positive values now fall back to the 16 KiB default like every other invalid input. Adds tests for the fraction, NaN, and mistyped max_bytes classes and for the mistyped compact argument's lenient false fallback.
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Included review availability: Your plan provides up to 8 included reviews per hour; 7 remain after this review. 📝 WalkthroughWalkthroughThe ChangesMCP context budget
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Severity of issue fixed: Medium Sequence Diagram(s)sequenceDiagram
participant MCPCaller
participant handleContext
participant FormatContextWithOptions
MCPCaller->>handleContext: Call mem_context with max_bytes and compact
handleContext->>FormatContextWithOptions: Pass ContextOptions
FormatContextWithOptions-->>handleContext: Return bounded context
handleContext-->>MCPCaller: Return result
Suggested reviewers: Merge Risk: ⚪ Minimal · up to The MCP context response now has bounded default output, capped pinned rows, optional compact rendering, and a clamped maximum budget. Current coverage validates the boundary and error behaviors, with no remaining concrete merge-blocking risk. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
🧪 Generate unit tests (beta)
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Inline comments:
In `@internal/mcp/mcp.go`:
- Around line 1833-1837: Update handleContext around FormatContextWithOptions so
the complete mem_context result, including stats.Projects and the nudge suffix,
never exceeds the requested max_bytes; reserve budget for or truncate these
appended sections before returning result. Extend the budget test in
internal/mcp/context_budget_test.go lines 150-164 with enough project names to
exercise the suffix and assert the full handler result remains within the
requested budget.
- Around line 1797-1801: Update the max_bytes parsing logic in
internal/mcp/mcp.go lines 1797-1801 to return memContextDefaultMaxBytes whenever
v has a fractional component, before converting it to int; retain the existing
fallback for values below 1. Add a context-budget test in
internal/mcp/context_budget_test.go lines 266-295 using a fractional value such
as 1.5 and assert that the default budget is returned.
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… budgets (Gentleman-Programming#1039) CodeRabbit follow-ups on PR Gentleman-Programming#1074: - Major: the byte budget now applies to the COMPLETE mem_context result, not just the context block. The stats suffix is rendered first and its bytes are reserved from the context budget, the projects join is capped at 8 names plus a +N more overflow marker (it was the unbounded part: a join over every project in the store), and a final UTF-8-safe clamp with the visible [truncated] marker backstops pathological cases where the suffix alone meets the budget. - Minor: fractional max_bytes values (1.5, 0.5, 2048.75) now fall back to the 16 KiB default instead of int-truncating into a budget the caller never requested. Also pins the max_bytes=1 minimum floor.
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Inline comments:
In `@internal/mcp/context_budget_test.go`:
- Around line 136-145: Extend the context handler tests to cover
loadContextStats failure: stub loadContextStats to return an error, register
restoration with t.Cleanup, invoke handleContext, and assert res.IsError plus
the returned error text. Keep the existing successful-statistics coverage
unchanged.
In `@internal/mcp/mcp.go`:
- Line 1917: Update the no-context response branch in the memory lookup flow to
pass the “No previous session memories found.” message through
clampMemContextResult before respondWithProject, preserving the requested
max_bytes budget; add a regression case covering a max_bytes value of 1 with no
matching memory.
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…udget (Gentleman-Programming#1039) CodeRabbit follow-up (second actionable) on PR Gentleman-Programming#1074: the fixed "No previous session memories found." message bypassed clampMemContextResult and escaped an explicit tiny max_bytes budget, breaking the complete-result bound the previous commit established. The message now flows through the same clamp, so max_bytes=1 with no matching memory returns a one-byte UTF-8-safe prefix instead of the full sentence. The default-budget case keeps the message verbatim (the clamp is a no-op under 16 KiB), pinned by the new regression test TestMemContextBudgetNoMemoryMessageBounded. The first actionable (loadContextStats failure-path coverage) needed no code: TestHandleContextPropagatesStatsError already stubs the error, restores with t.Cleanup, and asserts IsError plus the error text.
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Both actionables addressed in 3ffb443:
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Inline comments:
In `@internal/mcp/context_budget_test.go`:
- Around line 445-452: Update the max_bytes=1 assertion in the no-memory test to
require got == "N" instead of accepting any prefix, while preserving the
existing length and UTF-8 validity checks.
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…rogramming#1039) CodeRabbit follow-up on PR Gentleman-Programming#1074: the max_bytes=1 no-memory case now asserts got == "N" (the exact one-byte UTF-8-safe prefix) instead of accepting any prefix of the message, keeping the length and UTF-8 checks. Mirrors the R3 vacuous-assertion advisory from the native review of 3ffb443.
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Done in 70861a8: the max_bytes=1 no-memory case now asserts |
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Could someone with label access add I picked |
…eman-Programming#1039) Stub loadContextStats to fail inside the Gentleman-Programming#1039 budget suite and assert mem_context surfaces the error verbatim, closing CodeRabbit's unresolved minor on PR Gentleman-Programming#1074.
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@dnlrsls everything is resolved on my side and CI is green except one gate: this PR needs a
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🔗 Linked Issue
Closes #1039
🏷️ PR Type
type:bug— Bug fixtype:feature— New featuretype:docs— Documentation onlytype:refactor— Code refactoring (no behavior change)type:chore— Maintenance, dependencies, toolingtype:breaking-change— Breaking change📝 Summary
mem_contextMCP tool now renders a bounded context instead of the unbounded legacyFormatContextoutput: 16 KiB total budget by default (the feat(context): bound the SessionStart context injection #1012 Claude hook budget), 20 pinned rows, so the MCP path can never inject an unbounded payload into an agent's context window.max_bytesandcompacttool arguments mirroring the/contextquery params.max_bytesis clamped to the same 64 KiB ceiling the HTTP endpoint enforces; absent, mistyped, non-positive, NaN, or sub-integer input falls back to the bounded default, never to the unbounded legacy path.📂 Changes
internal/mcp/mcp.gohandleContextroutes throughFormatContextWithOptionswith budget constants and a float64-safememContextMaxBytesresolver; tool schema gainsmax_bytes/compactinternal/mcp/context_budget_test.gointernal/mcp/testdata/tool-contract-v1.jsoncompact(boolean) andmax_bytes(number) onmem_context🧪 Test Plan
go test ./...— all green exceptTestUnixSocketServesHTTPWithRestrictivePermissionsandTestUnixSocketCloseIsIdempotent, which fail identically on pristineorigin/mainon this machine (environmental, pre-existing)go test -tags e2e ./internal/server/...— not runnable here (same environmental Unix-socket failures as onorigin/main)make lint— golangci-lint is not available on this machine;go vet ./internal/...andgofmtare clean✅ Contributor Checklist
Closes #1039)type:*label to this PRgo test ./...go test -tags e2e ./internal/server/...make lintCo-Authored-Bytrailers in commits💬 Notes for Reviewers
gh pr edit --add-labeldoes not stick. Please addtype:feature(theperfcommit type maps to it per CONTRIBUTING).fix(mcp): keep fractional max_bytes on the bounded path) closes an edge found during review:int-truncation of a positive fraction below 1 producedMaxBytes=0, the store's unbounded-legacy sentinel.review-c5d66f2dc210046dcompleted approved (reliability lens); its three advisory suggestions are recorded as separate follow-up material, none blocking.Summary by CodeRabbit
mem_contexttool, with a 16 KiB default and 64 KiB maximum.max_bytesandcompactparameters for controlling output size and formatting.