Slice 1: Contamination fix — derive agent-file filter from tool filediff only

[jeonghun-jj-lee]

Important

Problem — The Files Changed tab's session.diff() handler builds its agent-touched-file filter from two sources: tool filediff metadata (clean — records exactly what the agent's edit/write/patch/apply_patch tools modified) and patch part file lists (contaminated — captures all worktree changes between step-start and step-finish snapshots, including modifications by other sessions or external processes). The contaminated source causes the tab to show files the current session never edited.

Approach — Remove the patch part file lists from the agentFilesAbsolute set in session.diff(). Derive the filter exclusively from tool filediff metadata. The patch parts retain their snapshot hashes (used for the from/to diff range) but their files arrays no longer feed the filter.

Scope — in: the session.diff() handler's file-filter loop, two new tests. out: the processor's patch-part emission (unchanged), the patch-part schema (unchanged), client-side code.

Acceptance Criteria

• A session with no edit/write/patch/apply_patch tool calls shows zero files in the Files Changed tab — the session.diff() endpoint returns an empty array
• A session with edit tool calls shows exactly the files those tools modified (deduplicated by path), with M/A/D status derived from the server's git-state comparison — not from patch-part file lists
• Concurrent sessions editing different files do not leak files into each other's Files Changed tabs
• Existing sessions with contaminated patch parts in the DB retroactively show correct (filtered) results

Testing Decisions

• Zero-edits test (new): construct a session with only read/grep/glob tool parts and patch parts containing file lists — assert session.diff() returns an empty array
• Contamination test (new): construct a session with patch parts listing files A, B, C alongside tool parts with filediff metadata for file A only — assert session.diff() returns only file A

Key Decisions

• Filter at the query handler, not the event emitter. The patch/step-end data in the processor is left unchanged — no backward compatibility concerns with existing DB records. The fix is a single loop change in session.diff().
• Snapshot hashes on patch parts still used. The from/to diff range computation reads step-start and step-finish snapshot hashes from their respective parts — these are independent of the patch.files array and remain the source of truth.

Constraints & Invariants

• The patch/step-end part schema is unchanged — no migration needed
• The diff query contract (GET /session/:id/diff) returns only agent-touched files, never workspace-wide changes
• The fallback chain (summary diffs, tool-part accumulation) continues to work for sessions lacking filediff metadata

Prior Art

• The session.diff() handler in the opencode server's session/session.ts (lines 838-998)
• Issue fix(app): remove per-message Changed files section from chat output #733: suppressed per-message Changed files for the same contamination (inline variant)
• PR fix(app): trust server session diff for Files Changed panel #720: client-side trust fix for the Files Changed panel

Source

Part of #741 · Follow-up to #733

[jeonghun-jj-lee]

Fixed in harmoniqs/opencode#285 — patch-part file lists removed from agent filter; now derived exclusively from tool filediff metadata.