[Gemini CLI] 400 Bad Request "Requests ending with a model turn are not supported" when reading image files

## Observed symptom bug(core): 400 Bad Request "Requests ending with a model turn are not supported" when reading image files via ReadFile tool ### What happened? When Gemini CLI attempts to inspect image files (e.g. `.png`) using the built-in `ReadFile` tool during an agentic session, the CLI encounters an unhandled API error and enters a broken state with repeated HTTP 400 errors: ```text ✓ ReadFile inspect_history_user.png → Read image file: inspect_history_user.png ✕ [API Error: [{ "error": { "code": 400, "message": "Requests ending with a model turn are not supported.", "errors": [ { "message": "Requests ending with a model turn are not supported.", "domain": "global", "reason": "badRequest" } ], "status": "INVALID_ARGUMENT" } } ]] ℹ T ## Literal error text ``` Requests ending with a model turn are not supported ``` ## Environment GitHub issue google-gemini/gemini-cli #29574 ## Trigger / context ### What happened? When Gemini CLI attempts to inspect image files (e.g. `.png`) using the built-in `ReadFile` tool during an agentic session, the CLI encounters an unhandled API error and enters a broken state with repeated HTTP 400 errors: ```text ✓ ReadFile inspect_history_user.png → Read image file: inspect_history_user.png ✕ [API Error: [{ "error": { "code": 400, "message": "Requests ending with a model turn are not supported.", "errors": [ { "message": "Requests ending with a model turn are not supported.", "domain": "global", "reason": "badRequest" } ], "status": "INVALID_ARGUMENT" } } ]] ℹ This request failed. Press F12 for diagnostics, or run /settings and change "Error Verbosity" to full for full details. ✓ ReadFile inspect_history_kim.png → Read image file: inspect_history_kim.png ✕ [API Error: [{ "error": { "code": 400, "message": "Requests ending with a model turn are not supported.", "errors": [ { "message": "Requests ending with a model turn are not supported.", "domain": "global", "reason": "badRequest" ## Source evidence - https://github.com/google-gemini/gemini-cli/issues/29574 (2026-10-01T03:30:57Z) — #29574 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: Ccac964845f ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.