feat(den-mcp): expose bounded input schemas in capability search and refuse un-constructible executes

[benjaminshafii]

Summary

search_capabilities on /mcp/agent returned external MCP tool matches with pathParams: [], queryParams: [], hasBody: true — the live inputSchema (which listExternalMcpTools already fetches, and which the admin route /v1/mcp-connections/:id/tools already returns) was discarded. The calling model had to guess tool arguments, producing empty/garbage payload calls at providers (the SyntaxError: Empty JSON string class of failures seen in an enterprise ServiceNow debugging session).

What changed

• inputSummary on external tool matches (searchExternalCapabilities): a bounded rendering of the tool's JSON schema — { required, properties: { name: { type, description, enum } }, truncated? }:
  • caps: 24 properties (required-first, rest alphabetical), 120-char descriptions, 8 enum values, ~1.5KB serialized total (shrinks property list until under budget); truncated: true whenever anything was cut;
  • shallow rendering: nested objects → "object", arrays → "array<primitive>"/"array"; defensive parsing with type guards throughout.
• schemaHash: sha256 of the recursively key-sorted schema JSON, first 12 hex chars — detects provider schema drift between search and execute.
• Execute guard: executeExternalCapability with an empty args object now pre-checks the live tool schema; if the tool declares non-empty required, it returns a structured missing_required_arguments error naming the fields and carrying inputSummary + schemaHash instead of forwarding an empty payload to the provider. The pre-check is best-effort (discovery failure falls through to the normal call path) and is skipped entirely when args are non-empty — zero added latency on the happy path.
• search_capabilities tool description mentions inputSummary.

Testing

• pnpm --filter @openwork-ee/den-api exec bun test test/external-capabilities-input-schema.test.ts — 5 pass (summary bounding + truncation flag, hash stability across key order, empty-args refusal with required fields and no provider call, empty-args passthrough when nothing required, non-empty args skip the pre-check).
• pnpm --filter @openwork-ee/den-api exec bun test test/external-capabilities-search-divergence.test.ts — 17 pass, 0 fail (unchanged behavior preserved).
• pnpm --filter @openwork-ee/den-api exec tsc --noEmit — clean.
• Results reproduced independently by a second run after implementation.

No video/fraimz attached: Den API/MCP surface change covered by the unit tests above. Reviewer repro: connect any external MCP whose tool declares required fields, run search_capabilities (observe inputSummary/schemaHash on the match), then execute_capability with an empty body (observe missing_required_arguments naming the fields, provider untouched).

Notes

• Complements the manual tool runner (feat(mcp): add manual tool runner #2752) and the diagnostics ledger (feat(mcp): stage the diagnostics verification release #2674): search now tells agents how to call a tool, and Den refuses calls it knows cannot be constructed.
• Additive optional fields on ExternalCapabilityMatch/ExternalCapabilityExecuteResult; no change to REST catalog matches.

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Project	Deployment	Actions	Updated (UTC)
openwork-app	Ready	Preview, Comment	Jul 14, 2026 8:52pm
openwork-den	Ready	Preview, Comment	Jul 14, 2026 8:52pm
openwork-den-worker-proxy	Ready	Preview, Comment	Jul 14, 2026 8:52pm
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