Detect changes in hosted content types in compaction contentEqual

[PratikDhanave (PratikDhanave)]

Problem

contentEqual (agent/compaction/equality.go) is the change-detection predicate that MessageIndex.Update uses (via contentsEqual/messageContentEqual) to locate the already-processed prefix, and that containsMessageByContent uses for source attribution. Its switch handles text/reasoning/data/uri/error/function-call/function-result/hosted-file and returns true (equal) for everything else.

So two messages that differ only in a registered hosted content type — MCPServerToolCallContent, MCPServerToolResultContent, CodeInterpreterToolCallContent, CodeInterpreterToolResultContent, or UsageContent — are reported equal. MessageIndex.Update then treats the changed message as unchanged and does not rebuild, preserving stale group/exclusion state and mis-locating the prefix boundary. The existing TestMessageIndexUpdate_MatchesKnownContentTypes encodes the intent that any differing value must trigger a rebuild; the default violated it for these types.

Fix

Add explicit equality cases for the hosted content types the framework actually produces, comparing their identity fields (recursing into Outputs/Inputs via the existing contentsEqual, and reflect.DeepEqual for UsageDetails). The default intentionally stays return true so a genuinely new, forward-compatible content type does not force an index rebuild — only the known hosted types get precise comparison.

Test

Extends TestMessageIndexUpdate_MatchesKnownContentTypes with rows for each hosted type: identical values still preserve the index; differing Arguments/Outputs/Inputs/usage now correctly rebuild. The differing rows fail before the fix (treated as equal) and pass after.

[Copilot]

🟡 Changes recommended

The new contentsEqual(leftContent.Outputs/Inputs, ...) calls use message.Contents where contentsEqual expects []message.Content, which will not compile without explicit slice conversions.

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Pull request overview

This PR fixes change detection in agent/compaction by teaching contentEqual how to compare several hosted/registered content types that were previously treated as equal via the default branch, which could cause MessageIndex.Update to preserve stale indexing state when hosted tool/usage content changes.

Changes:

• Added explicit equality handling for MCPServerTool*, CodeInterpreterTool*, and UsageContent in contentEqual.
• Kept default: return true for forward-compatible unknown content types, while ensuring known hosted types are compared precisely.
• Extended TestMessageIndexUpdate_MatchesKnownContentTypes with new rows covering these hosted types.

File summaries

File	Description
agent/compaction/equality.go	Adds explicit equality cases for hosted tool-call/tool-result, code-interpreter, and usage content in contentEqual.
agent/compaction/equality_test.go	Extends the existing compaction equality/update test table with cases for the hosted content types.

Review details

Suppressed comments (1)

agent/compaction/equality.go:87

• Inputs/Outputs are message.Contents, but contentsEqual takes []message.Content; these calls need an explicit conversion to avoid a compile error.

	case *message.CodeInterpreterToolCallContent:
		rightContent := right.(*message.CodeInterpreterToolCallContent)
		return leftContent.CallID == rightContent.CallID && contentsEqual(leftContent.Inputs, rightContent.Inputs)
	case *message.CodeInterpreterToolResultContent:
		rightContent := right.(*message.CodeInterpreterToolResultContent)
		return leftContent.CallID == rightContent.CallID && contentsEqual(leftContent.Outputs, rightContent.Outputs)

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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[github-actions]

Scope: internal-only

Changed Go contract: None. contentEqual, messageContentEqual, contentsEqual, and errorsEqual in agent/compaction/equality.go are all unexported. No exported types, functions, methods, fields, options, or documented behavior in agent/compaction's public surface change as a result of this PR.

Upstream evidence reviewed: Not applicable — this PR only extends an existing unexported switch in contentEqual with explicit comparison cases for *message.MCPServerToolCallContent, *message.MCPServerToolResultContent, *message.CodeInterpreterToolCallContent, *message.CodeInterpreterToolResultContent, and *message.UsageContent, following the exact same pattern already used for the adjacent *message.FunctionResultContent/*message.HostedFileContent cases in the same function. It corrects a bug (these hosted types previously fell through to the catch-all return true) rather than introducing new behavior, naming, or defaults that would require comparison against .NET/Python.

Result: out of scope — this is a self-contained internal bugfix restoring intended change-detection semantics within MessageIndex.Update's existing prefix-matching logic (already exercised by TestMessageIndexUpdate_MatchesKnownContentTypes). No public API or cross-language-observable behavior is added or altered, so no parity comparison applies. No public-api-change label needed.

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