fix(mcp): forward image tool results on the existing image channel - #989
fix(mcp): forward image tool results on the existing image channel#989cairn-intern wants to merge 4 commits into
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Part 1 of Gitlawb#823 named dropped non-text blocks. Screenshot servers still could not hand the model the picture. Decode MCP image blocks onto tools.Result.Images so the agent loop can emit them, and only name the block types still not forwarded. Fixes Gitlawb#823
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Included review availability: Your plan provides up to 4 included reviews per hour; 0 remain after this review. WalkthroughMCP tool results now decode valid image payloads, enforce per-image and aggregate size limits, and forward them through ChangesMCP image forwarding
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to This localized change forwards MCP image results through the existing image channel without any actionable merge-blocking risk remaining beyond normal checks and review. Sequence Diagram(s)sequenceDiagram
participant MCPServer
participant registryTool
participant forwardImages
participant ResultImages
MCPServer->>registryTool: return text and image content
registryTool->>forwardImages: decode and validate payloads
forwardImages-->>registryTool: image blocks and drop dispositions
registryTool->>ResultImages: forward valid images
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✅ Passed checks (4 passed)
Full details: Linked Issues checkExplanation The changes satisfy issue Full details: Out of Scope Changes checkExplanation The modified client, registry logic, public content fields, and tests directly support MCP image forwarding, aggregate size limits, drop reporting, and single-pass conversion. No unrelated changes are evident. ✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@internal/mcp/client.go`:
- Around line 568-575: Update ImageBlocks to enforce an aggregate limit on
forwarded images, such as a total byte or image-count bound, while preserving
per-image validation from imageBlockFromContent. Stop or skip images once the
aggregate limit is reached, and record any otherwise-valid images rejected by
that limit in the dropped-content summary using the existing summary mechanism.
In `@internal/mcp/non_text_content_test.go`:
- Around line 264-284: The test coverage in TestMalformedImageDataDoesNotPanic
should also exercise oversized-image rejection: provide a valid, decodable image
payload larger than imageinput.MaxImageBytes, then assert Result.Images is empty
and result.Output identifies the dropped image.
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internal/mcp/client.gointernal/mcp/non_text_content_test.gointernal/mcp/registry.go
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ImageBlocks applied MaxImageBytes per block only, so many 10 MiB images could exhaust memory. Cap the sum at MaxImageBytes and skip the next valid image once it would exceed the remaining budget. DroppedContentSummary now omits only images ImageBlocks actually kept, so aggregate-skipped payloads are named rather than silently dropped.
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gnanam1990
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Verdict
Changes requested. Reviewed head 8ca707c47b29b2de5b698c5dc8951ca0c4468077 against merge base 27b319ca88a3180bed5183f0c599e9307f3ece12.
No new third-party module, dependency, SDK, service, provider, vendor tree, submodule, or remote runtime integration is introduced by this PR.
[Medium] The aggregate cap bounds retained images but decodes every accepted image three times
registryTool.Run first calls ImageBlocks. It then calls DroppedContentSummary, which calls ImageBlocks again and subsequently calls imageBlockFromContent once more for every non-text item while matching forwarded blocks. A valid image is therefore base64-decoded and allocated three times in one tool call. Images rejected by the aggregate budget are also repeatedly decoded even though their bytes will never be forwarded.
This leaves the new 10 MiB aggregate budget as a retention limit, not a work/allocation limit. A disposable benchmark on this exact head measured:
- one 5 MiB image:
ImageBlocksalone allocated 5,251,120 B/op;registryTool.Runallocated 15,753,696 B/op (three repeat runs produced the same byte counts), - four 5 MiB images:
registryTool.Runallocated 63,021,088 B/op even though only the 10 MiB aggregate is retained.
That payload fits comfortably under the existing 64 MiB stdio MCP frame cap, so a buggy or hostile server can trigger this through the normal tool-result path; HTTP/SSE responses make bounding decode work at this layer at least as important.
Please convert content once and return both the accepted ImageBlocks and per-item forwarding/drop disposition to the caller. Build the summary from that disposition instead of decoding again, and stop decoding once no aggregate budget remains. Add a regression benchmark or an injectable decode counter proving one decode per candidate and bounded work after the budget is exhausted.
What I verified
go test ./internal/mcp -count=1passed.- Focused MCP image tests passed under
-race. - Agent tool-result image delivery and OpenAI/Anthropic/Gemini image mapping tests passed.
make fmt-check,go vet ./..., focused package vet, andgit diff HEAD --checkpassed.- A full
go test ./...run reached and passed the touched MCP/agent/provider-image paths, but the repository-wide run did not finish cleanly because untouched CLI observability tests failed and an unrelated OpenAI retry test hit the 3-minute timeout.
The benchmark was added only in a disposable review worktree and removed afterward. The PR branch was not modified, and the head was re-confirmed immediately before this review.
registryTool.Run called ImageBlocks then DroppedContentSummary, which decoded every accepted image two more times and kept decoding after the aggregate budget was spent. Classify content in one pass, build the drop note from that disposition, and skip later image payloads once no budget remains.
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Addressed in c259851.
Existing behavior is unchanged: forwarded images still ride
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Vasanthdev2004
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Requesting changes. Your CI had never run: the checks were held at action_required behind the fork gate, so the lone green check was CodeRabbit. I released it and the full suite is green, so the two things below are from reading and probing rather than from a red build.
An image-only result delivers an empty tool_result to the model. The new guard if output == "" && len(images) == 0 leaves Output empty when the only content is a successfully forwarded image, and nothing downstream substitutes: ModelOutput() returns it verbatim and finalizeToolOutcome copies it through. So the happy path for the feature this PR adds hands the model an empty text body alongside the image. A one-line placeholder ("[image forwarded]" or similar) keeps the result self-describing.
An image dropped by the aggregate budget is described as unrecoverable. forwardImages marks a budget-skipped image as dispDropped, which puts it in the same sentence as audio and resource blocks: "which Zero cannot forward yet. Retrying cannot recover this payload." For a budget drop that is wrong twice over, because Zero can forward it and a retry with fewer images would recover it. Worth a distinct message so the model does not give up on a payload it could get.
One note, not blocking: the gate is remaining > 0, so any residue leaves it open and each later candidate is fully base64-decoded before the length check rejects it. The doc gloss calling the cap a work limit is only exactly true when the budget lands on zero. It is one clause on a wrapper with no production caller, so I would fix the sentence rather than the code.
… note An image-only tool result left Output empty, so the model got a blank tool_result next to the image. Budget-skipped images reused the unrecoverable drop sentence even though a retry with fewer images would recover them.
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Addressed in dff11d0. Image-only success now sets Budget-skipped images get their own disposition (
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jatmn
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I found issues that need to be addressed before this is ready.
Merge readiness
- [P1] Rebase onto current
mainbefore merge
internal/mcp/oauth.go
The PR head is based on27b319ca, while livemainhas advanced through1b5db176with a newer MCP OAuth change. GitHub currently reports the branch as mergeable, but this repository treats a stale base as a review blocker; rebase and re-check the resolved diff before merging.
Findings
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[P1] Do not send MCP screenshots to a model that cannot accept images
internal/mcp/registry.go:330
forwardImagesaccepts the block here andResult.Imagescarries it into the agent loop. The loop then always turns it into a following user message, and every provider mapper serializes that message as multimodal input; none of those paths checks the active model. This bypasses the existing CLI and TUI policy, which explicitly discard direct image input whenmodelregistry.SupportsVisioncannot confirm support. As a result, a text-only or unknown/custom model can call a screenshot MCP tool successfully, receive the normal textual result, and then have its next completion rejected solely because the newly added image part was sent.Please address the root cause at the common tool-result delivery boundary: make the effective model’s vision capability available where tool images are converted into the following user message, and drop/notice unsupported attachments there while preserving the tool’s text output. Cover both a vision-capable model (image is delivered) and a non-vision/unknown model (text continues, image is not sent). This should apply uniformly to MCP and existing image-producing tools, rather than adding an MCP-only provider workaround.
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[P2] Accept a padded image exactly at the documented 10 MiB limit
internal/mcp/client.go:652
The earlyDecodedLencheck treats an upper bound as an exact size.MaxImageBytesis 10,485,760 (one modulo three), so an image exactly at the documented inclusive cap is encoded with==padding:DecodedLenreports 10,485,762, whileDecodeStringproduces exactly 10,485,760 bytes. The function returns at line 652, never reaches its correct post-decodelen(data) > MaxImageBytescheck, and reports a valid at-limit image as unforwardable. The existing file-image boundary uses the inclusive>rule, so this also makes equivalent image inputs disagree at the limit.Please keep the fail-closed, pre-allocation protection but make it padding-aware (or otherwise use a safe encoded-length threshold that cannot exclude an input decoding to exactly the cap). Retain the exact post-decode backstop, and add regressions for both an exactly-
MaxImageBytesstandard-base64 PNG with==padding andMaxImageBytes + 1; the former must forward and the latter must remain dropped.
4 participants
Part 2 of #823: actually forward MCP image tool-result blocks on the existing
tools.Result.Imageschannel.#874 (merged) was part 1 only — it names dropped non-text blocks and does not carry the payload. #843 added
tools.Result.Imagesfor builtin capture tools; the agent loop already emits those as a following user message. This PR does not duplicate either: it fills that channel from MCPtype: imagecontent so a screenshot server can hand the model the picture.Changes
typeimage, typicalmimeTypeimage/png, base64data) ontoContent.Data. Absentdatastill unmarshals (backward compatible).[]zeroruntime.ImageBlockthe same way capture tools do: cap atimageinput.MaxImageBytes(10 MiB), sniff withhttp.DetectContentType+zeroruntime.NormalizeImageMediaType(png/jpeg/gif/webp only).registryTool.RunsetsResult.Imagesfrom forwarded image blocks.DroppedContentSummaryskips blocks that were successfully forwarded, so the note no longer says “cannot forward yet” for images that were forwarded. Audio/resource/structured (and failed-decode images) still get the existing drop note.Imagesset. Empty text + images does not become(empty MCP tool result).Tests
Added/updated in
internal/mcp/non_text_content_test.go(same package style,t.Fatalf, no testify):Result.Images; drop summary empty for that casedataplus compatibility whendatais absentgo testwas not run against a full checkout (git API + box files +gofmtonly). CI should rungo test ./internal/mcp -count=1.Issue is issue-approved. Linux-only is fine.
Fixes #823
Summary by CodeRabbit
New Features
Bug Fixes