fix(request-filter): track header modifications for proper display in session details (#457)

[ding113]

Summary

Fixed issue where request filter modifications to headers (especially user-agent) were not reflected in Session detail pages.

Problem

Users reported that when setting headers via Request Filter (e.g., SET user-agent), the modified headers were visible in trace logs but not in the Session detail page's request headers section.

Root Cause

1. ProxySession.fromContext()
   ├─ headers: new Headers(c.req.header())  ← Original
   ├─ userAgent: headers.get("user-agent") ← Read-only snapshot
   └─ headerLog: formatHeadersForLog()     ← For logging

2. Guard Pipeline → RequestFilterGuard
   └─ session.headers.set("user-agent", "custom")  ← Modified

3. ProxyForwarder.buildHeaders()
   ├─ overrides["user-agent"] = session.userAgent || fallback  ← Problem: uses read-only snapshot
   └─ Ignores filtered values in session.headers

Solution

Added header modification tracking to detect when filters change headers:

Changes

src/app/v1/_lib/proxy/session.ts

• Added originalHeaders field to snapshot headers at session creation
• Added isHeaderModified(key) method to detect changes
• Preserves empty strings vs null correctly

src/app/v1/_lib/proxy/forwarder.ts

• Modified Codex user-agent resolution logic:
  • Check session.isHeaderModified("user-agent") first
  • Use ?? instead of || to preserve empty strings
  • Priority: filtered UA > original UA > fallback
• Improved debug logging (removed potential sensitive data leak)

tests/unit/proxy/session.test.ts

• Added 6 test cases for isHeaderModified()
• Coverage: modify/unmodify/delete/add/empty-string/null cases

tests/unit/proxy/proxy-forwarder.test.ts

• Added 5 test cases for user-agent resolution
• Coverage: all priority paths + empty string edge case

Test Results

✓ All type checks passed
✓ All lint checks passed
✓ 22/22 unit tests passed

Test Coverage

• isHeaderModified(): 6 tests (100% branch coverage)
• buildHeaders(): 5 tests (100% branch coverage)
• Edge cases: empty string UA, header deletion, header addition

Edge Cases Fixed

1. Empty string UA: Now correctly preserved (was falling back incorrectly)
2. Header deletion: Detected and falls back to original UA as expected
3. Header addition: Detected as modification
4. Logging security: Replaced value logging with length to prevent info leak

Backward Compatibility

✅ Fully backward compatible

• New field is private (originalHeaders)
• New method is additive (isHeaderModified())
• No database migrations required
• No configuration changes needed

Verification Steps

To manually verify:

1. Create a Request Filter rule: SET user-agent = "Test-UA/1.0"
2. Send a Codex format request
3. Check Session detail page → Request Headers
4. Verify user-agent shows as "Test-UA/1.0"

Closes #457

🤖 Generated with Claude Code

Greptile Summary

Fixed request filter header modifications not being reflected in upstream requests by implementing header modification tracking. The solution adds an originalHeaders snapshot in ProxySession and an isHeaderModified() method to detect when filters change headers. The buildHeaders() method in ProxyForwarder now checks for modifications and prioritizes filtered values over the cached session.userAgent property.

Key Changes:

• Added originalHeaders field to capture initial header state at session creation
• Implemented isHeaderModified(key) to detect header changes by comparing original vs current values
• Updated Codex user-agent resolution logic to respect filter modifications
• Changed from || to ?? operator to properly preserve empty string values
• Improved debug logging to avoid potential sensitive data leaks (logs length instead of value)

Architecture:
The fix correctly handles the execution order where guards (client, version) validate using original headers before filters run, while buildHeaders() uses filtered headers for upstream requests. This maintains proper separation of concerns.

Test Coverage:
11 new test cases provide complete coverage of modification detection and user-agent resolution priority paths, including edge cases like empty strings and header deletion.

Confidence Score: 5/5

• This PR is safe to merge with minimal risk - the implementation is clean, well-tested, and backward compatible
• Score reflects excellent implementation quality: proper encapsulation with private originalHeaders field, comprehensive test coverage (11 tests covering all edge cases), correct use of null coalescing operator to preserve empty strings, improved security in logging, and backward compatibility with no breaking changes. The fix correctly handles the execution flow where validation guards use original headers while forwarding uses filtered headers.
• No files require special attention - all changes are well-implemented and thoroughly tested

Important Files Changed

Filename	Overview
src/app/v1/_lib/proxy/session.ts	Added originalHeaders snapshot and isHeaderModified() method to track filter modifications - clean implementation with proper encapsulation
src/app/v1/_lib/proxy/forwarder.ts	Updated Codex user-agent resolution to prioritize filtered headers using modification tracking - logic is correct with proper null coalescence
tests/unit/proxy/session.test.ts	Added 6 comprehensive test cases for isHeaderModified() covering all edge cases (modify, delete, add, empty string)
tests/unit/proxy/proxy-forwarder.test.ts	Added 5 test cases for user-agent resolution covering filtered/original/fallback priority and empty string preservation

Sequence Diagram

sequenceDiagram
    participant Client
    participant ProxySession
    participant GuardPipeline
    participant RequestFilter
    participant ProxyForwarder
    participant UpstreamProvider

    Client->>ProxySession: Request with user-agent: "Original-UA/1.0"
    ProxySession->>ProxySession: Create session<br/>headers = new Headers(req.headers)<br/>originalHeaders = new Headers(headers)<br/>userAgent = headers.get("user-agent")
    
    ProxySession->>GuardPipeline: Execute guards
    GuardPipeline->>GuardPipeline: auth, client, version guards<br/>(use session.userAgent = "Original-UA/1.0")
    
    GuardPipeline->>RequestFilter: Execute request filter
    RequestFilter->>ProxySession: session.headers.set("user-agent", "Filtered-UA/2.0")
    Note over ProxySession: headers modified<br/>originalHeaders unchanged<br/>userAgent unchanged
    
    RequestFilter-->>GuardPipeline: Filter complete
    GuardPipeline-->>ProxySession: All guards passed
    
    ProxySession->>ProxyForwarder: Forward request
    ProxyForwarder->>ProxyForwarder: buildHeaders()
    ProxyForwarder->>ProxySession: isHeaderModified("user-agent")?
    ProxySession-->>ProxyForwarder: true (original ≠ current)
    
    ProxyForwarder->>ProxySession: headers.get("user-agent")
    ProxySession-->>ProxyForwarder: "Filtered-UA/2.0"
    
    ProxyForwarder->>ProxyForwarder: resolvedUA = filteredUA ?? originalUA ?? fallback<br/>= "Filtered-UA/2.0"
    
    ProxyForwarder->>UpstreamProvider: Request with user-agent: "Filtered-UA/2.0"
    UpstreamProvider-->>ProxyForwarder: Response
    ProxyForwarder-->>Client: Response

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[gemini-code-assist]

Summary of Changes

Hello @ding113, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where modifications to request headers, particularly the User-Agent, made by request filters were not accurately reflected in the session detail pages or correctly propagated to downstream services. The solution involves implementing a robust header modification tracking system within the session object and updating the header forwarding logic to respect these tracked changes, thereby ensuring data consistency and correct behavior across the proxy.

Highlights

• Header Modification Tracking: Introduced a mechanism to track header modifications made by request filters, ensuring that changes (like to the User-Agent) are correctly reflected in session details and subsequent processing.
• Codex User-Agent Resolution Logic: Updated the ProxyForwarder to prioritize User-Agent values from request filters, then the original User-Agent, and finally a fallback, correctly handling cases like empty strings and deletions.
• Enhanced Debug Logging: Improved debug logging for User-Agent resolution by logging value lengths instead of the actual values, preventing potential sensitive data leaks.
• Comprehensive Unit Tests: Added extensive unit tests for both the isHeaderModified() method in ProxySession (6 cases) and the User-Agent resolution logic in ProxyForwarder (5 cases), covering various edge cases.

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[gemini-code-assist]

Code Review

This is a great fix that correctly addresses the issue of header modifications not being reflected. The introduction of originalHeaders and isHeaderModified is a clean solution. The unit tests are comprehensive and cover all the relevant edge cases, which is excellent.

I've left one suggestion to simplify the user-agent resolution logic in ProxyForwarder, which makes the code a bit more concise without changing its behavior.

Overall, a solid contribution that improves correctness and is well-tested.

[gemini-code-assist]

The logic for resolving the User-Agent can be simplified. The if (wasModified) block is redundant because the logic in both branches can be combined into a single expression.

When wasModified is false, it means the current header value (filteredUA) is the same as the original (originalUA). In this scenario, the expression from the if branch (filteredUA ?? originalUA ?? fallback) simplifies to originalUA ?? fallback, which is identical to the else branch.

Because both cases can be covered by the same logic, you can replace the entire if/else block. This makes the code more concise and declarative.

      const resolvedUA =
        filteredUA ?? originalUA ?? "codex_cli_rs/0.55.0 (Mac OS 26.1.0; arm64) vscode/2.0.64";

[github-actions]

🧪 测试结果

测试类型	状态
代码质量	✅
单元测试	✅
集成测试	✅
API 测试	✅

总体结果: ✅ 所有测试通过

[github-actions]

Code Review Summary

No significant issues identified in this PR.

PR Size: M

• Lines changed: 288
• Files changed: 4

Review Coverage

• Logic and correctness - Clean
• Security (OWASP Top 10) - Clean
• Error handling - Clean
• Type safety - Clean
• Documentation accuracy - Clean
• Test coverage - Adequate
• Code clarity - Good

---

Automated review by Codex AI

[github-actions]

• Reviewed PR #465 (4 files changed, 288 lines changed) against the 6 required perspectives, scoped strictly to diff lines.
• Applied the size label size/M.
• Ran npx @biomejs/biome check . to validate lint status (pass).
• No issues met the >=80 confidence reporting threshold, so no inline diff comments were posted.
• Submitted the required summary review comment to the PR via gh pr review.

[github-actions]

Code Review Summary

This PR implements header modification tracking to fix issue #457 where request filter modifications to headers were not reflected in session detail pages. The implementation is clean, well-tested, and follows the project architecture patterns.

PR Size: M

• Lines changed: 288 (282 additions, 6 deletions)
• Files changed: 4

Issues Found

Category	Critical	High	Medium	Low
Logic/Bugs	0	0	0	0
Security	0	0	0	0
Error Handling	0	0	0	0
Types	0	0	0	0
Comments/Docs	0	0	0	0
Tests	0	0	0	0
Simplification	0	0	0	0

Analysis Summary

Strengths:

1. Correct architecture: The solution properly separates concerns by snapshotting original headers at session creation and tracking modifications via isHeaderModified(). This allows validation guards to use original headers while forwarding uses filtered headers.

2. Proper null handling: Using ?? instead of || in buildHeaders() (forwarder.ts:1863) correctly preserves empty string user-agent values, fixing a real edge case bug.

3. Security improvement: Replacing session.userAgent value logging with length logging prevents potential sensitive data leakage in debug logs (forwarder.ts:1873).

4. Encapsulation: The originalHeaders field is properly marked as private readonly, maintaining good encapsulation (session.ts:56-57).

5. Comprehensive testing: 11 new test cases cover all branches including edge cases (empty strings, header deletion, header addition). Tests verify behavior rather than implementation.

6. Accurate documentation: The JSDoc comment for isHeaderModified() (session.ts:211-223) accurately describes the method's behavior including edge cases.

Code Quality Observations:

• The logic correctly implements the priority chain: filtered UA > original UA > fallback
• The isHeaderModified() method correctly detects modifications by comparing original vs current values, returning true for deletions (null !== original), additions (original !== null), and value changes
• Test helpers like createSessionForHeaders() properly initialize both headers and originalHeaders to maintain the invariant that they start identical

No Issues Found:
The implementation is production-ready with no bugs, security vulnerabilities, type safety issues, or documentation errors identified. The code adheres to CLAUDE.md guidelines and follows existing patterns in the codebase.

Review Coverage

• Logic and correctness - Clean
• Security (OWASP Top 10) - Clean (improved logging security)
• Error handling - Clean (no error handling code added, no issues)
• Type safety - Clean (proper private readonly encapsulation)
• Documentation accuracy - Clean (JSDoc accurate)
• Test coverage - Excellent (100% branch coverage for new code)
• Code clarity - Good

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Automated review by Claude AI