OpenCode threads fail on Markdown file attachments

[iamhenry]

What happened

BB accepts a Markdown attachment when creating an OpenCode thread, but the turn fails before inference with:

'file part media type text/markdown' functionality not supported.

Later text-only messages such as resume fail with the same error, so the thread cannot recover normally.

What should happen

OpenCode threads should receive supported file input, such as a staged file-path text marker, or BB should reject unsupported attachments before starting the turn with an actionable message. A rejected attachment should not prevent later text-only turns from running.

Repro

1. Create a BB thread with the OpenCode provider.
2. Attach a .md file with MIME type text/markdown.
3. Send the message.
4. Observe that the turn fails immediately.
5. Send a text-only follow-up.
6. Observe the same attachment error again.

Evidence

• Reproduced in thread thr_7cyd92n6md on BB 0.42.2-nightly.34486466131.1.
• Each failed turn reports the exact error above with no model output.
• packages/provider-bridge-acp/src/bridge/bridge.ts:1290-1296 converts every localFile to an ACP resource_link without provider capability handling.
• packages/provider-bridge-acp/src/bridge/bridge.test.ts has no localFile or resource_link coverage.
• Codex and Pi bridges already use path-bearing text markers for localFile, which provides a compatible fallback pattern.

Acceptance

• Attaching a Markdown file to an OpenCode thread does not fail the turn.
• Unsupported file inputs produce a clear preflight error or a readable path marker.
• A failed attachment does not poison later text-only turns.
• Tests cover initial and follow-up OpenCode turns with a Markdown attachment.

AGENT GENERATED

[bb-slop-cop]

🚨 SLOP COP 🚨 · new-issue-autopilot

Applied Bug · Medium priority · Medium effort, plus providers, provider-acp, and opencode. No existing classification values were replaced.

Reproduction report — PARTIALLY REPRODUCED, medium root-cause confidence; applied partial-repro.

The ACP bridge unconditionally emits a resource link for local files without a readable text fallback. Two clean checkouts reproduced the missing fallback in initial and follow-up attachment tests. The full build passed. Real OpenCode inference rejection and persistent text-only failures remain unverified because OpenCode was unavailable here.

No PR: this partial reproduction does not satisfy the rule’s verified-bug gate for a safe fix.

AGENT GENERATED

[SawyerHood]

Consolidating #3500 into this earlier tracker at the owner’s request during the past-week duplicate review.

Same incident and acceptance criteria. Preserve the clarification that the reporter uses a custom OpenCode provider plugin. Both independent reports confirm the missing ACP attachment fallback, but real MIME rejection and persistent text-only failure remain unverified.

The original issue and its comments remain available through the link. Closing the duplicate does not mean this work is fixed or complete.

AGENT GENERATED