fix(mcp): return isError=True on tool failures

[shard-c6]

Fixes #2714

[graphify-labs]

Graphify reviewed this change.

Looks safe to merge — no coupling regressions and no blocking issues, checked against the code graph (not a self-assessment).

Formal verification. No changes could be formally verified in this run.

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Graphify review — findings

This PR changes the call_tool function in graphify/serve.py to return a types.CallToolResult object directly instead of a raw list of TextContent. In the error and unknown-tool paths, it now sets isError=True on the returned result. The _on_call_tool handler is updated accordingly to pass through the result rather than wrapping the previous list return value.

No blocking issues surfaced. 3 lower-confidence candidates did not survive cross-model review.

Analysis details — impact, health, verification

Impact & health

Graphify review

Impact — 435 functions depend on the 114 functions this change touches.

Health — grade A; 10 existing hotspot(s) in the area this change touches (pre-existing, not introduced here):

• dispatch_command() — 2 callers, 117 callees (high)
• _query_graph_text() — 18 callers, 8 callees (high)
• _score_query() — 15 callers, 5 callees (high)
• _query_terms() — 17 callers, 3 callees (high)
• run_benchmark() — 16 callers, 3 callees (high)
• _build_server() — 2 callers, 16 callees (high)
• _load_graph() — 9 callers, 3 callees (medium)
• _query_subgraph_tokens() — 7 callers, 3 callees (medium)
• …and 2 more

Verification — 435 functions in the blast radius were not formally verified this run (proofs are advisory here).

Gate & verification

graphify gate

PASS — objectively clean (no health regressions, tests not run — proofs not run this pass (advisory)). Grounded, not self-assessed.

Advisory (not blocking):

• verification_scope: 285 function(s) in the blast radius were not formally verified this run

Formal verification

Could not verify: Could not verify \_build\_server.

The verifier did not have enough to check \_build\_server, so it is saying so rather than guessing. No false assurance is the whole point.

Guarantee: No guarantee either way, this is an honest abstention, not a pass.

Note: Reason: not verifiable: all 23 sampled inputs raised on both versions — the function never executed, so 'no divergence' would be vacuous (mostly ImportError — names the real obstacle, not a sampling gap)