400 "unknown variant `image_url`, expected `text`" from text-only OpenAI-compatible providers when history contains images — poisons every subsequent turn

Environment

• Kimi Code CLI 0.33.0, Linux (WSL2, Ubuntu 24.04)
• Custom OpenAI-compatible provider in config.toml: type = "openai", base_url = "https://api.deepseek.com/v1", model declared with capabilities = [ "thinking", "tool_use" ] (no vision), used as [secondary_model]

Steps to reproduce

1. Work in a session whose history contains image blocks (e.g. results of the built-in media-read tool, or pasted images).
2. Switch the session model to the text-only model (/model), or route a turn to it as the configured secondary model.
3. Send any message.

Actual behavior

Every request fails with:

[provider.api_error] 400 Failed to deserialize the JSON body into the target type:
messages[119]: unknown variant `image_url`, expected `text`

Kimi Code replays the history verbatim, so the provider (whose chat-completions schema only accepts text content parts for this model) rejects the whole payload. Because the image block stays in history, every subsequent turn fails the same way — the session is effectively bricked until the wire file is hand-patched.

Expected behavior

When the resolved model has no vision capability (Kimi Code already knows the capability set from the model declaration / catalog), image (and other non-text) content parts should be stripped or converted to a text placeholder before the request is sent, instead of being forwarded verbatim.

Notes

• Same failure class as #1082 (failed image re-sent every turn deadlocks the session) and related to #905 (pasted images on text-only models) — but here the trigger is history replay to a custom OpenAI-compatible text-only provider via /model switch or secondary-model routing, and the error surfaces as a provider-side deserialization 400.
• Current workarounds on our side: only run text-only models in fresh sessions without images, and a maintenance script that rewrites image blocks to text in wire.jsonl to unbrick affected sessions.

Start at the /model switch and secondary-model routing, then trace how image-containing history from wire.jsonl is replayed to a resolved model without vision. Reproduce the provider-side deserialization failure and verify that subsequent turns no longer resend unsupported image parts; the fix should preserve usable text history for text-only providers.