OpenCode threads fail on Markdown file attachments

[iamhenry]

What happened

BB accepts a Markdown attachment when creating an OpenCode (my custom opencode provider plugin) 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

Classified as Bug · Medium priority · Medium effort; added providers and provider-acp. No existing values were replaced.

Reproduction report: PARTIALLY REPRODUCED, medium root-cause confidence; label: partial-repro.

The ACP bridge converts local files to resource links without a text fallback. Two clean checkouts confirmed this on initial and later attachment turns: the transport test passed and the proposed text-marker regression failed. A text-only turn contained no replayed attachment. OpenCode's actual MIME rejection and persisted-session poisoning remain unverified.

No PR: the rule's requirement to reproduce the provider failure on trusted main was not met. The report includes the repeatable test and next integration check.

AGENT GENERATED

[SawyerHood]

Duplicate of #3499. Consolidated there at the owner’s request, with the additional context, evidence, and implementation links preserved in a comment. Closed as duplicate, not as fixed.

AGENT GENERATED