fix: make OutputFormat schemas encodable from persisted plain objects (#26929)

[padraig-myers-sh]

Issue for this PR

Closes#26929

Type of change

• Bug fix

What does this PR do?

GET /session/:id/message returns HTTP 400 whenever a session contains a message with format.type = "json_schema":

Expected OutputFormatJsonSchema, got {"type":"json_schema","schema":{...},"retryCount":2}
at [0]["info"]["format"]

OutputFormatText and OutputFormatJsonSchema were declared with Schema.Class. An Effect Schema.Class only encodes genuine class instances. When a format object is persisted onto a user message and read back from MessageTable, the row data holds a plain object (message-v2.tsinfo() spreads row.data). The GET endpoint validates its response against Schema.Array(SessionV1.WithParts), so re-encoding the stored format through Schema.Class fails — one bad message rejects the entire array with a 400.

Converting both to Schema.Struct makes encode accept plain objects. All decode validation is preserved (negative retryCount still rejected, json_schema without schema still rejected). Neither schema is constructed with new() or checked with instanceof anywhere, so dropping class identity is safe.

How did you verify your code works?

Added 4 encode regression tests in packages/opencode/test/session/structured-output.test.ts:

• Encodes text format from a plain object
• Encodes json_schema format from a plain object with retryCount
• Encodes json_schema format from a plain object without retryCount
• Round-trips json_schema through decode then encode

Without the fix, 3 of 4 new tests fail. With the fix, all 26 tests pass:

bun test packages/opencode/test/session/structured-output.test.ts
26 pass
0 fail
57 expect() calls

Also verified end-to-end in our downstream product (StubHub Shift): the GET endpoint was returning 400 on every structured-output turn, causing cost/usage tracking to report $0. After applying this patch, the 400 is gone and usage data is correctly reported.

Screenshots / recordings

N/A — no UI changes.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential duplicate found:

• PR fix(session): encode persisted output formats #37541: fix(session): encode persisted output formats
  fix(session): encode persisted output formats #37541

This PR appears to be addressing the same issue: encoding persisted output formats that are read back as plain objects. It likely attempted a similar fix for the GET /session/:id/message HTTP 400 error when handling json_schema formats.

You should review PR #37541 to understand what approach was taken previously and why the current fix (switching from Schema.Class to Schema.Struct) may be different or necessary.

[padraig-myers-sh]

Closing as duplicate of #37541 — same fix (Schema.Class → Schema.Struct), same root cause, same issue (#26929). PR #37541 also exports the types and includes an endpoint-level regression test. Apologies for the noise.