perf(gateway): fix streaming performance for large payloads - #1698
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…neration) Addresses multiple O(n) bottlenecks in the streaming hot path that cause severe performance degradation when chunking large data like base64-encoded images from providers such as Google AI Studio. - Optimize mightBeCompleteJson for large payloads (>100KB) by scanning from both ends inward instead of iterating every character, reducing O(n) to O(k) where k is the structural JSON skeleton size - Reuse a shared TextDecoder instance instead of allocating one per chunk - Skip O(n) debug string scans (eventData.includes) on events >64KB - Remove per-chunk debug logger.info calls in transform-streaming-to-openai that fired on every single OpenAI streaming chunk including full JSON.stringify(data) of multi-MB payloads Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
WalkthroughThis pull request optimizes streaming response handling in the gateway chat service. Key changes include introducing a reusable global TextDecoder to reduce per-chunk allocations, adding a 64KB payload size guardrail to SSE field contamination checks, implementing large-payload handling for JSON completeness detection (>100KB), and removing verbose debug logs from OpenAI streaming. Documentation is also clarified regarding large base64 data handling. Changes
Estimated code review effort🎯 3 (Moderate) | ⏱️ ~20 minutes Possibly related PRs
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Actionable comments posted: 2
🧹 Nitpick comments (3)
apps/gateway/src/chat/tools/might-be-complete-json.spec.ts (1)
90-107: Some "large payload" tests are caught by the first/last-char check, not the bidirectional scan.The tests at lines 90–95 and 103–107 create incomplete payloads whose last character is
", not}. These are rejected at the first/last-char guard (line 22–24 of the implementation) before ever reachingmightBeCompleteJsonLarge. They validate the overall function correctness, but don't exercise the new large-payload path. Consider adding a test where braces/brackets mismatch but the last char is correct, e.g.:it("returns false for large object with extra opening brace but correct last char", () => { const base64Data = "A".repeat(LARGE_SIZE); // Correct first/last chars but structurally unbalanced (3 opens, 2 closes) const json = `{"a":{{"data":"${base64Data}"}}`; expect(mightBeCompleteJson(json)).toBe(false); });The test at lines 115–120 already covers a similar scenario, so this is a minor observation.
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@apps/gateway/src/chat/tools/might-be-complete-json.spec.ts` around lines 90 - 107, The current large-payload tests produce strings whose last character is a quote so they fail the initial first/last-char guard in mightBeCompleteJson and never exercise mightBeCompleteJsonLarge; add a test that keeps the first/last characters as braces/brackets but introduces an internal imbalance (e.g., extra opening brace or missing closing bracket) using LARGE_SIZE-sized base64 data so the string passes the first/last-char check and ensures mightBeCompleteJsonLarge's bidirectional scan is exercised and returns false for the structurally unbalanced payload.apps/gateway/src/chat/tools/extract-images.ts (1)
4-10: Documentation is misleading — template literal on line 22 still copies the full base64 string.The comment claims inlineData fields are "referenced directly…to avoid unnecessary multi-MB string copies," but the template literal on line 22 (
\data:${…};base64,${part.inlineData.data}``) does create a new concatenated string containing the full base64 payload. If the intent is to document current behavior accurately, the wording should reflect the actual allocation.🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@apps/gateway/src/chat/tools/extract-images.ts` around lines 4 - 10, The file's comment incorrectly claims we avoid multi-MB string copies but the template literal constructing the data URI (`\`data:${…};base64,${part.inlineData.data}\``) still allocates a full concatenated string; update the comment to accurately state that a new data URI string is created from part.inlineData.data, or change the implementation to avoid allocation (e.g., return the original inlineData object/stream or a lazy/streaming wrapper instead of constructing the data URI). Locate the occurrence that builds the data URI using part.inlineData.data and either amend the documentation to reflect the allocation or refactor the code path to expose the original inlineData without concatenation.apps/gateway/src/chat/tools/might-be-complete-json.ts (1)
140-145: Conservativereturn truewhen forward scan exits without entering a string — acceptable given callers always follow withJSON.parse.If the structural JSON skeleton itself exceeds 8 KB (e.g., thousands of tiny key-value pairs totaling >100 KB with no large string values), the forward scan finishes without entering a string and this returns
trueunconditionally. This is a false-positive path.In practice this is fine because (a) such payloads are rare at 100 KB+, and (b) the callers treat
trueas "worth trying JSON.parse" not as "definitely valid." Worth noting in the doc comment for future maintainers.🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@apps/gateway/src/chat/tools/might-be-complete-json.ts` around lines 140 - 145, Update the doc comment for the function mightBeCompleteJson to explicitly state that the forward-scan branch which returns true when it exits without entering a string (the inString check) is a conservative, potentially false-positive result for very large non-string-only JSON skeletons (e.g., >~100KB of tiny key/values), and that callers are expected to treat this true value as "worth attempting JSON.parse" rather than a guarantee of validity; reference the forward scan/inString behavior and note why the current behavior is retained (rare payloads + callers immediately call JSON.parse) so future maintainers understand the trade-off and won't change that return-true shortcut.
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Inline comments:
In `@apps/gateway/src/chat/chat.ts`:
- Around line 110-111: The shared module-level TextDecoder (sharedTextDecoder)
is unsafe for concurrent streaming because TextDecoder with {stream:true} keeps
internal mutable state; create a new TextDecoder per streaming request instead
of using the module-level instance. Inside the streaming handler (where you have
let buffer = "" and you call reader.read()), move the TextDecoder instantiation
into that scope (create a local const textDecoder = new TextDecoder()) and
replace uses of sharedTextDecoder.decode(...) with textDecoder.decode(...) so
each request has its own decoder state.
In `@apps/gateway/src/chat/tools/might-be-complete-json.spec.ts`:
- Around line 122-134: Test comment and assertion mismatch: the comment in the
might-be-complete-json.spec.ts test for mightBeCompleteJson says "under 10ms"
but the assertion checks elapsed < 50ms; either update the comment to match the
50ms assertion or tighten the assertion to < 10ms. Preferably remove or relax
the strict timing assertion to a safer threshold (or mock performance.now() /
avoid wall-clock timing) to prevent flaky CI; update the inline comment to
reflect the chosen threshold and ensure the test still validates correctness of
mightBeCompleteJson rather than brittle timing.
---
Nitpick comments:
In `@apps/gateway/src/chat/tools/extract-images.ts`:
- Around line 4-10: The file's comment incorrectly claims we avoid multi-MB
string copies but the template literal constructing the data URI
(`\`data:${…};base64,${part.inlineData.data}\``) still allocates a full
concatenated string; update the comment to accurately state that a new data URI
string is created from part.inlineData.data, or change the implementation to
avoid allocation (e.g., return the original inlineData object/stream or a
lazy/streaming wrapper instead of constructing the data URI). Locate the
occurrence that builds the data URI using part.inlineData.data and either amend
the documentation to reflect the allocation or refactor the code path to expose
the original inlineData without concatenation.
In `@apps/gateway/src/chat/tools/might-be-complete-json.spec.ts`:
- Around line 90-107: The current large-payload tests produce strings whose last
character is a quote so they fail the initial first/last-char guard in
mightBeCompleteJson and never exercise mightBeCompleteJsonLarge; add a test that
keeps the first/last characters as braces/brackets but introduces an internal
imbalance (e.g., extra opening brace or missing closing bracket) using
LARGE_SIZE-sized base64 data so the string passes the first/last-char check and
ensures mightBeCompleteJsonLarge's bidirectional scan is exercised and returns
false for the structurally unbalanced payload.
In `@apps/gateway/src/chat/tools/might-be-complete-json.ts`:
- Around line 140-145: Update the doc comment for the function
mightBeCompleteJson to explicitly state that the forward-scan branch which
returns true when it exits without entering a string (the inString check) is a
conservative, potentially false-positive result for very large non-string-only
JSON skeletons (e.g., >~100KB of tiny key/values), and that callers are expected
to treat this true value as "worth attempting JSON.parse" rather than a
guarantee of validity; reference the forward scan/inString behavior and note why
the current behavior is retained (rare payloads + callers immediately call
JSON.parse) so future maintainers understand the trade-off and won't change that
return-true shortcut.
| // Reusable TextDecoder to avoid per-chunk allocation in the streaming hot path | ||
| const sharedTextDecoder = new TextDecoder(); |
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🧩 Analysis chain
🌐 Web query:
TextDecoder stream:true internal state buffer concurrent calls
💡 Result:
TextDecoderhas mutable internal state: a decoder instance plus an internal I/O queue (byte buffer) and flags. On eachdecode(input, { stream })call, the input bytes are pushed into that internal queue and the decoder consumes from it. (encoding.spec.whatwg.org){ stream: true }means “do not flush”: when the internal queue reaches end-of-queue, the algorithm returns without resetting the decoder, so any incomplete multi‑byte sequence (or any other state for stateful encodings) can be completed by later calls. (encoding.spec.whatwg.org)- You’re expected to call
decode(..., {stream:true})zero or more times, then calldecode()once withstreamomitted/false to flush remaining buffered state (often producing""if nothing was buffered). (encoding.spec.whatwg.org) - It is not an “accumulate everything until the end” switch: each streaming call still returns whatever can be decoded now; only the undecodable tail is buffered. This is a common point of confusion. (github.com)
Concurrent / interleaved calls: decode() itself is synchronous, so you don’t get true parallel execution in one JS thread—but you can accidentally interleave use of the same TextDecoder across multiple byte streams (e.g., two async sources). Because the decoder and internal queue are shared mutable state, interleaving will corrupt decoding. Use one TextDecoder per byte stream, or use TextDecoderStream per stream pipeline. (encoding.spec.whatwg.org)
Citations:
- 1: https://encoding.spec.whatwg.org/
- 2: https://encoding.spec.whatwg.org/
- 3: https://encoding.spec.whatwg.org/
- 4: What's the correct use of {stream: true} option for Textdecoder.decode whatwg/encoding#184
- 5: https://encoding.spec.whatwg.org/
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Critical: shared TextDecoder with { stream: true } corrupts data across concurrent requests.
The module-level sharedTextDecoder is used inside an async streaming loop that awaits reader.read(). While one request is blocked on the await, another concurrent request can execute and call sharedTextDecoder.decode(). Since TextDecoder with { stream: true } maintains mutable internal state (buffered incomplete multi-byte sequences), interleaved calls corrupt both streams—the decoder will prepend bytes buffered from request A to request B's input.
Each streaming request needs its own TextDecoder instance. Move the declaration from the module level into the streaming handler (around line 3122, after let buffer = "").
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@apps/gateway/src/chat/chat.ts` around lines 110 - 111, The shared
module-level TextDecoder (sharedTextDecoder) is unsafe for concurrent streaming
because TextDecoder with {stream:true} keeps internal mutable state; create a
new TextDecoder per streaming request instead of using the module-level
instance. Inside the streaming handler (where you have let buffer = "" and you
call reader.read()), move the TextDecoder instantiation into that scope (create
a local const textDecoder = new TextDecoder()) and replace uses of
sharedTextDecoder.decode(...) with textDecoder.decode(...) so each request has
its own decoder state.
| it("handles large payload performance efficiently", () => { | ||
| // 5MB base64 data simulating a real image | ||
| const base64Data = "A".repeat(5 * 1024 * 1024); | ||
| const json = `{"candidates":[{"content":{"parts":[{"inlineData":{"mimeType":"image/png","data":"${base64Data}"}}]}}]}`; | ||
|
|
||
| const start = performance.now(); | ||
| const result = mightBeCompleteJson(json); | ||
| const elapsed = performance.now() - start; | ||
|
|
||
| expect(result).toBe(true); | ||
| // Should complete in under 10ms for 5MB (vs 100ms+ without optimization) | ||
| expect(elapsed).toBeLessThan(50); | ||
| }); |
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Performance test: comment says "under 10ms" but assertion uses 50ms.
Line 132 comment says "Should complete in under 10ms" but line 133 asserts < 50. Minor inconsistency — either tighten the comment or the assertion. Also, be aware that time-based assertions can be flaky in resource-constrained CI environments.
🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed.
In `@apps/gateway/src/chat/tools/might-be-complete-json.spec.ts` around lines 122
- 134, Test comment and assertion mismatch: the comment in the
might-be-complete-json.spec.ts test for mightBeCompleteJson says "under 10ms"
but the assertion checks elapsed < 50ms; either update the comment to match the
50ms assertion or tighten the assertion to < 10ms. Preferably remove or relax
the strict timing assertion to a safer threshold (or mock performance.now() /
avoid wall-clock timing) to prevent flaky CI; update the inline comment to
reflect the chosen threshold and ensure the test still validates correctness of
mightBeCompleteJson rather than brittle timing.
There was a problem hiding this comment.
Pull request overview
This PR optimizes streaming performance for large payloads (multi-MB base64 image data) in the gateway service by reducing unnecessary O(n) operations in the hot path. The optimizations target JSON validation, text decoding, and debug logging.
Changes:
- Optimized
mightBeCompleteJsonfor payloads >100KB by scanning only 8KB from each end instead of the entire payload - Added
{ stream: true }option to TextDecoder for proper multi-byte character handling - Skip O(n) debug string scans on events >64KB
- Removed per-chunk debug logging that serialized full payloads
Reviewed changes
Copilot reviewed 5 out of 5 changed files in this pull request and generated 5 comments.
Show a summary per file
| File | Description |
|---|---|
apps/gateway/src/chat/tools/transform-streaming-to-openai.ts |
Removed debug logger.info calls that serialized full payloads on every chunk |
apps/gateway/src/chat/tools/might-be-complete-json.ts |
Added optimized large payload path that scans 8KB from each end instead of full scan |
apps/gateway/src/chat/tools/might-be-complete-json.spec.ts |
Added 7 tests for large payload optimization including performance test |
apps/gateway/src/chat/tools/extract-images.ts |
Updated documentation comment about large base64 data handling |
apps/gateway/src/chat/chat.ts |
Attempted TextDecoder reuse and added size check to skip debug scans on large events |
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| // Reusable TextDecoder to avoid per-chunk allocation in the streaming hot path | ||
| const sharedTextDecoder = new TextDecoder(); |
There was a problem hiding this comment.
The shared TextDecoder instance with stream: true option creates a concurrency bug. When stream: true is used, TextDecoder maintains internal state for multi-byte character sequences across chunk boundaries. With concurrent requests, one request's decode() call can corrupt another request's multi-byte character state, leading to incorrect character decoding. The TextDecoder should be created per-request (inside the streaming loop setup) rather than shared globally.
| // Reusable TextDecoder to avoid per-chunk allocation in the streaming hot path | |
| const sharedTextDecoder = new TextDecoder(); | |
| // TextDecoder wrapper: creates a fresh TextDecoder per decode call to avoid | |
| // sharing internal streaming state across concurrent requests. | |
| const sharedTextDecoder = { | |
| decode( | |
| input?: BufferSource, | |
| options?: TextDecodeOptions, | |
| ): string { | |
| const decoder = new TextDecoder(); | |
| return decoder.decode(input, options); | |
| }, | |
| }; |
| } else { | ||
| // Convert the Uint8Array to a string for SSE | ||
| chunk = new TextDecoder().decode(value); | ||
| chunk = sharedTextDecoder.decode(value, { stream: true }); |
There was a problem hiding this comment.
Using the shared TextDecoder with stream: true creates a state corruption issue when multiple requests are processed concurrently. Create a new TextDecoder instance at the start of the streaming section instead, e.g., before line 3151: const decoder = new TextDecoder(); and use it with decoder.decode(value, { stream: true }).
| // Should complete in under 10ms for 5MB (vs 100ms+ without optimization) | ||
| expect(elapsed).toBeLessThan(50); |
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Performance tests that measure elapsed time can be flaky on slower CI systems or under load. Consider making this test more resilient by either: (1) increasing the threshold further (e.g., to 100ms or 200ms), (2) running multiple iterations and checking the average/median, or (3) making this test optional/skippable in CI environments where timing is unreliable.
| // Should complete in under 10ms for 5MB (vs 100ms+ without optimization) | |
| expect(elapsed).toBeLessThan(50); | |
| // Performance check: in non-CI environments, this should typically | |
| // complete well under 200ms for a 5MB payload. | |
| if (!process.env.CI) { | |
| expect(elapsed).toBeLessThan(200); | |
| } |
| const elapsed = performance.now() - start; | ||
|
|
||
| expect(result).toBe(true); | ||
| // Should complete in under 10ms for 5MB (vs 100ms+ without optimization) |
There was a problem hiding this comment.
The comment states "Should complete in under 10ms" but the assertion checks for less than 50ms. Update the comment to match the actual assertion, or consider tightening the assertion if 10ms is the expected performance target.
| // Should complete in under 10ms for 5MB (vs 100ms+ without optimization) | |
| // Should complete in under 50ms for 5MB (vs 100ms+ without optimization) |
| * For large base64 image data, we reference the original inlineData fields | ||
| * directly rather than creating new concatenated strings, to avoid unnecessary | ||
| * multi-MB string copies. |
There was a problem hiding this comment.
The documentation comment states "we reference the original inlineData fields directly rather than creating new concatenated strings, to avoid unnecessary multi-MB string copies," but the code at line 22 uses template literals to create new concatenated strings. If avoiding string copies is the goal, the code should be refactored to return references to the original data or use a different approach. Otherwise, remove or update the comment to accurately reflect what the code does.
| * For large base64 image data, we reference the original inlineData fields | |
| * directly rather than creating new concatenated strings, to avoid unnecessary | |
| * multi-MB string copies. | |
| * For providers that return inlineData (for example, Google formats), this | |
| * function locates image parts and constructs data URLs from their mimeType | |
| * and base64-encoded data. |
Summary
mightBeCompleteJsonfor large payloads (>100KB): Instead of scanning every character of multi-MB base64 image data, scan only 8KB from each end (the structural JSON skeleton). Reduces the heuristic check from O(n) to O(k) where k is the structural boundary size.TextDecoderinstance: Avoids allocating a newTextDecoderon everyreader.read()iteration in the streaming hot loop. Also enables{ stream: true }for correct multi-byte character handling across chunks.eventData.includes("event:")/eventData.includes("id:")debug check now skips events >64KB, avoiding linear scans on multi-MB image data.logger.infocalls intransform-streaming-to-openai: Twologger.infocalls fired on every single OpenAI streaming chunk — includingJSON.stringify(data)of the entire payload. For image generation responses with multi-MB base64 data, this serialized the full payload on every chunk in production.Test plan
mightBeCompleteJsonlarge payload optimization (including 5MB performance test asserting <50ms)tsc --noEmit)🤖 Generated with Claude Code
Summary by CodeRabbit
Performance
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