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## Problem
`buzz-dev-mcp` advertises `view_image` to every agent regardless of
whether the session's model accepts images. When a text-only model (e.g.
DeepSeek V4 Flash) takes the bait, the image lands in session history
and every subsequent LLM request 404s with `No endpoints found that
support image input`. The error was classified as `LlmModelNotFound` and
propagated fatally out of the turn loop — history stays poisoned,
buzz-acp retries the batch with exponential backoff, and the session
burns its entire clock doing no work. In a recent trial run, **all 57
trials that called `view_image` on a text-only model died this way; none
recovered.**
## Fix
Capability-gating the advertised tool isn't reliable — there is no
image-capability metadata at the agent layer across providers. Instead,
recover at the turn loop:
- **Typed error**: new `AgentError::UnsupportedImageInput`, classified
narrowly on the exact provider phrase `No endpoints found that support
image input` on both the generic 404 path and OpenRouter's 404 path.
Unknown-model 404s and OpenRouter parameter-routing 404s keep their
existing classifications. No deterministic retry.
- **In-turn recovery**: on this error, `RunCtx::run` strips every image
block from history — keeping the tool result (and therefore
tool-call/result pairing) intact — marks the result `is_error`, appends
actionable model-facing guidance ("The current model does not support
image input. The image was removed from conversation history so this
turn can continue. Use a text-based inspection tool…"), and continues
the same turn. Base64 never replays again.
- **Loop guard**: recovery only fires when at least one image was
removed; if the provider says "image" and history has none, the error
propagates as before.
## Tests
- Unit: phrase classification (typed, not retried; unknown-model 404
unaffected), idempotent image-to-error history mutation preserving call
IDs and text.
- End-to-end (`fake_llm.rs` + `fake_mcp.rs`): tool call → MCP image
result → 404 unsupported-image → same-turn recovery. Captured requests
prove round 2 carried the image, round 3 replays no image, carries the
guidance text, preserves pairing, and ends `end_turn`.
- Loop guard: typed unsupported-image error with **no** image in history
fails after exactly one provider request instead of spinning —
mutation-testing showed deleting the `removed == 0` guard survived the
suite, and `max_rounds` defaults to unlimited in production, so this
branch needed direct coverage.
Verified at `a210305019b33d5f56677b4c82bab79e4ac52d24`: `cargo test -p
buzz-agent` (full package, 381 unit + all integration suites) green;
`clippy --all-targets -D warnings` green; `fmt --check` green; pre-push
hooks (rust-tests, desktop-tauri-checks, branch-skew) green.
**Scope of the classification guarantee**: the classifier runs in the
shared `post()` (which Anthropic and OpenAI paths route through) and in
`openrouter_post()` — i.e., every 404 path in `llm.rs`. It only runs on
404 responses; providers that reject images with a different status
(e.g. a 400) are out of scope for this PR — see the review-comment
discussion for why broadening the phrase list alone would not cover
them.
Authored by Wren, loop-guard test by Sami, reviewed by Eva.
---------
Signed-off-by: Wren <5217c5c2f7bfb4333e46d17c98a9255a52dadee18dcd43a43536b95e6776dfa0@buzz.block.builderlab.xyz>
Signed-off-by: Sami <f4a42a97e594b77bdbd8ee35191c8b28a94a4cb871d96f32921558275421fb68@buzz.block.builderlab.xyz>
Co-authored-by: Wren <5217c5c2f7bfb4333e46d17c98a9255a52dadee18dcd43a43536b95e6776dfa0@buzz.block.builderlab.xyz>
Co-authored-by: Sami <f4a42a97e594b77bdbd8ee35191c8b28a94a4cb871d96f32921558275421fb68@buzz.block.builderlab.xyz>