Compare all identity fields in function call/result equality

[PratikDhanave]

Problem

contentEqual (agent/compaction/equality.go) is a structural-equality predicate, and every arm compares all the fields that distinguish two content values — TextReasoningContent compares ProtectedData, ErrorContent compares Message/ErrorCode/Details, DataContent compares all three fields, etc.

The function-content arms were the exception, ignoring fields the types actually carry:

case *message.FunctionCallContent:
    // drops Error and InformationalOnly
    return l.CallID == r.CallID && l.Name == r.Name && l.Arguments == r.Arguments
case *message.FunctionResultContent:
    // drops Error
    return l.CallID == r.CallID && reflect.DeepEqual(l.Result, r.Result)

The most consequential omission is FunctionResultContent.Error — the field that distinguishes a failed tool call from a successful one. Two results with the same CallID and Result but different Error compared equal.

MessageIndex.Update uses this predicate to decide whether the previously-processed prefix is unchanged (the early-return at index.go:81). So a tool result that flips between failure and success at the conversation tail can be treated as already-processed and skipped during re-indexing.

Fix

Compare the missing fields. Errors are compared by message string via a small errorsEqual helper, matching how FunctionCallContent/FunctionResultContent already serialize their Error field to and from JSON (Go error values aren't generally ==-comparable).

Test

Extends the existing TestMessageIndexUpdate_MatchesKnownContentTypes table (black-box, in equality_test.go) with the previously-uncovered fields: a FunctionCallContent differing only in InformationalOnly, one differing only in Error, and a FunctionResultContent differing only in Error — each expected not to match. All three fail on main (got true want false) and pass with the fix; the existing rows are unchanged.

[Copilot]

Pull request overview

This pull request fixes a correctness gap in agent/compaction’s structural content equality so that tool/function call and result content comparisons account for all identity-defining fields, preventing MessageIndex.Update from incorrectly treating changed tool outcomes as unchanged.

Changes:

• Extend contentEqual to compare FunctionCallContent.Error, FunctionCallContent.InformationalOnly, and FunctionResultContent.Error.
• Add a small errorsEqual helper that compares error values by their message string.
• Expand the existing TestMessageIndexUpdate_MatchesKnownContentTypes table to cover the newly-compared fields.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
agent/compaction/equality.go	Makes function call/result structural equality include previously-ignored fields (notably Error).
agent/compaction/equality_test.go	Adds regression tests ensuring changes in InformationalOnly / Error prevent equality matches.

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