feat(mcp): per-call result size override via _meta annotation#6343
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MCP servers can now request a larger truncation threshold for a single tool call by annotating the result with _meta["zeph/maxResultSizeChars"], clamped server-side to the new [tools.overflow].max_per_call_override ceiling (default 128K). Absent or malformed annotations fall back to the existing global threshold unchanged. Includes a config migration (step 89) so existing on-disk configs surface the new key.
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Summary
_meta["zeph/maxResultSizeChars"], mirroring Claude Code's_meta["anthropic/maxResultSizeChars"]pattern.[tools.overflow].max_per_call_override(default 131072 = 128 KiB) — the MCP server can never exceed this ceiling and can only raise the effective limit above the existing globalthreshold, never lower it._metavalues fall back to exactly the prior global-threshold behavior (fail-closed, zero regression for servers that don't opt in).max_per_call_override = 0disables the feature entirely and logs atracing::warn!if misconfigured (0 < max_per_call_override <= threshold).migrate_overflow_max_per_call_override) so existing on-disk configs surface the new key via--migrate-config.Closes #3079
Design
Full architecture review chain: architect → critic (approved) → developer → tester/perf/security/impl-critic (parallel validation) → code review (2 rounds). See
.local/handoff/in this branch's history for the complete design and audit trail.Notable findings resolved during review:
config/default.tomlwas initially missed (onlycrates/zeph-core/config/default.tomlgot the new key) — caught by code review, fixed.zeph-sanitizer's independentmax_content_size(64 KiB default) further bounds the real effective ceiling below the documented 128 KiB.Test plan
_metaparsing (missing/non-numeric/negative/float/bool/overflow), M2 misconfiguration warning, E2EToolOutput→process_one_tool_resultpath--profile ci, nextest full workspace, rustdoc gate, gitleaks) — clean.local/testing/playbooks/mcp.mdupdated with 4 new scenarios (valid override, above-ceiling clamp, malformed/missing fallback, disabled-via-zero).local/testing/coverage-status.mdrow added (Status: Untested — pending live-session verification)