What
Vertex-served older models rejected the tool-search beta header; those sessions now fall back to loading MCP tools normally instead of deferring them behind ToolSearch.
Details
- applies only when the provider is vertex, and matches claude-3* models plus opus/sonnet/haiku models outside generations 4 and 5
- short-circuits the tool-search capability check with reason
vertex_model_unsupported - the same check is applied at the MCP-deferral, tool-search-tool-injection and headless MCP prewait call sites
- the periodic tool-search reminder is suppressed for those models
Evidence
Tool search disabled for model '${e}' on Vertex: this model's Vertex serving stack rejects the tool-search beta header (pre-4.5 generation)., this model's Vertex serving stack rejects the tool-search beta header (pre-4.5 generation).
Strings lifted out of the shipped bundle, so the claim above can be checked against them.