[MAabdelmoumen]

Fixes the case where an agent that asks for JSON output gets prose instead.

The problem

is_single_string_output treats one string output as the agent's free text and skips structured generation. It cannot distinguish two different intents, because both produce an identical outputs list:

text agent              outputs: [{ title: "output",    type: "string" }]
JSON, one string field  outputs: [{ title: "sentiment", type: "string" }]

So a caller declaring a one-field JSON schema gets no structured-output tool in the request at all — the model is never told a schema was wanted, and answers in prose. Adding a second field changes nothing about the model, endpoint or prompt, yet works.

Verified against a vLLM deployment (nvidia/GLM-5.3-NVFP4) which supports json_schema, json_object and forced tool calls:

schema	tool in request	result
1 field, String	none	prose
1 field, Enum	none	prose
2 fields	AgentOutputModel	JSON, both fields

In a flow it is worse than cosmetic: the declared property is absent, so the downstream node raises Expected node ... to have a value for property ..., but none was found and the flow dies.

The change

Gate on the caller's declared format when one is given. metadata["outputFormat"] carries TEXT or JSON.

When no format is declared the previous shape heuristic still applies, so existing callers keep today's behaviour.

Why weak models stay safe

The heuristic exists so a string output works on models without structured-output support. That protection is unchanged: a single string output still satisfies is_single_string_output, so extract_outputs_from_invoke_result still falls back to the final message when structured generation returns nothing. The worst case is the prose returned today — never an empty output.

Note that oracle/agent-spec main has no such heuristic; it always builds the response model and forces ToolStrategy. This moves the fork back toward that behaviour for callers that declare a format.

Refs OA-1681

[MAabdelmoumen]

Closing — wrong base. This was branched from c83e113 (the commit agentfabric pins), which is not an ancestor of main, so the diff swept in unrelated commits. Reopening against the correct base shortly.