fix(mcp): truncate tool names exceeding 64-char provider limit

[georgeglarson]

Issue for this PR

Closes #3523

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

OpenAI's tool-use API rejects tools[].function.name strings longer than 64 characters. When an MCP server's combined <sanitized-server>_<sanitized-tool> overflows, the provider returns 400 and opencode silently stops processing because the error is logged at DEBUG but never surfaces to the TUI — exactly the symptom in the issue.

buildToolName() (new helper next to sanitize in packages/opencode/src/mcp/index.ts) returns combined names ≤64 chars unchanged. Overflows truncate to prefix(55) + "_" + hash(8) where hash is Hash.fast(combined).slice(0, 8). That preserves 55 chars of readable prefix while keeping keys distinct when two tools' first 64 chars would otherwise collide. Every truncation logs a log.warn.

Applied at tools() only — the call site feeding function.name to the provider. The :-separated keys in fetchFromClient() (prompts/resources) aren't subject to the 64-char limit, so I left them alone.

PR #15595 attempted this but was opened against the pre-Effect namespace MCP code, so that diff is rebase-incompatible regardless of approach. It also uses blind .slice(0, 64), which silently drops one of any two tools whose first 64 chars match — the hash suffix here prevents that.

How did you verify your code works?

Two new tests in packages/opencode/test/mcp/lifecycle.test.ts:

1. Registers chrome-devtools-aaaaaaaaaaaaaaaaaaa (the issue's reproducer) with two tools whose combined names share a 55-char prefix. Asserts both end up in the registry, both ≤64 chars, both end with _[0-9a-f]{8}, and the keys are distinct.
2. Short-name case asserts the key is unchanged <server>_<tool>.

Demonstrating the bug catch with the source change reverted (tests kept):

Run	Combined name length	Test result
Without this fix	82 chars	expect(<= 64), got 82 — fail
With this fix	64 chars w/ _<8hex> suffix	pass

Full run: bun test test/mcp/lifecycle.test.ts → 21 pass, 0 fail (19 existing + 2 new). bun run typecheck → clean.

Screenshots / recordings

N/A — no UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate found:

• PR fix: truncate MCP tool names exceeding 64 characters #15595: "fix: truncate MCP tool names exceeding 64 characters"
  • fix: truncate MCP tool names exceeding 64 characters #15595

Why it's related:
PR #15595 is explicitly mentioned in your PR description as a previous attempt to fix the same issue (truncating MCP tool names exceeding the 64-character limit). However, #15595 was opened against pre-Effect code and used a blind .slice(0, 64) approach that could silently drop tools with matching prefixes. Your PR (#24871) provides an improved implementation using a hash suffix to prevent collisions.

[Copilot]

Pull request overview

Fixes MCP tool registration against OpenAI’s 64-character tools[].function.name limit by introducing a deterministic truncation strategy that preserves uniqueness, preventing provider 400s that previously caused MCP processing to stop silently.

Changes:

• Add buildToolName() helper to cap <server>_<tool> names at 64 chars using a readable prefix plus an 8-hex hash suffix on overflow.
• Apply buildToolName() in MCP.tools() when constructing the tool map passed to the provider.
• Add lifecycle tests covering truncation behavior and collision avoidance, plus a short-name “unchanged” case.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
packages/opencode/src/mcp/index.ts	Introduces buildToolName() and uses it when registering tools to enforce the 64-char provider constraint.
packages/opencode/test/mcp/lifecycle.test.ts	Adds tests ensuring long tool names are truncated to ≤64 with unique hashed suffixes, and short names remain unchanged.

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