feat(hooks): add read-image-resizer hook

Intercepts Read tool output with image attachments and resizes to comply with Anthropic API limits (≤1568px long edge, ≤5MB). Only activates for Anthropic provider sessions and appends resize metadata (original/new resolution, token count) to tool output.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

src/config/schema/hooks.ts

@@ -49,6 +49,7 @@ export const HookNameSchema = z.enum([
   "write-existing-file-guard",
   "anthropic-effort",
   "hashline-read-enhancer",
+  "read-image-resizer",
 ])
 
 export type HookName = z.infer<typeof HookNameSchema>

src/hooks/index.ts

@@ -50,3 +50,4 @@ export { createRuntimeFallbackHook, type RuntimeFallbackHook, type RuntimeFallba
 export { createWriteExistingFileGuardHook } from "./write-existing-file-guard";
 export { createHashlineReadEnhancerHook } from "./hashline-read-enhancer";
 export { createJsonErrorRecoveryHook, JSON_ERROR_TOOL_EXCLUDE_LIST, JSON_ERROR_PATTERNS, JSON_ERROR_REMINDER } from "./json-error-recovery";
+export { createReadImageResizerHook } from "./read-image-resizer"

src/hooks/read-image-resizer/hook.test.ts

@@ -0,0 +1,286 @@
+/// <reference types="bun-types" />
+
+import { beforeEach, describe, expect, it, mock } from "bun:test"
+import type { PluginInput } from "@opencode-ai/plugin"
+
+import type { ImageDimensions, ResizeResult } from "./types"
+
+const mockParseImageDimensions = mock((): ImageDimensions | null => null)
+const mockCalculateTargetDimensions = mock((): ImageDimensions | null => null)
+const mockResizeImage = mock(async (): Promise<ResizeResult | null> => null)
+const mockGetSessionModel = mock((_sessionID: string) => ({
+  providerID: "anthropic",
+  modelID: "claude-sonnet-4-6",
+} as { providerID: string; modelID: string } | undefined))
+
+mock.module("./image-dimensions", () => ({
+  parseImageDimensions: mockParseImageDimensions,
+}))
+
+mock.module("./image-resizer", () => ({
+  calculateTargetDimensions: mockCalculateTargetDimensions,
+  resizeImage: mockResizeImage,
+}))
+
+mock.module("../../shared/session-model-state", () => ({
+  getSessionModel: mockGetSessionModel,
+}))
+
+import { createReadImageResizerHook } from "./hook"
+
+type ToolOutput = {
+  title: string
+  output: string
+  metadata: unknown
+  attachments?: Array<{ mime: string; url: string; filename?: string }>
+}
+
+function createMockContext(): PluginInput {
+  return {
+    client: {} as PluginInput["client"],
+    directory: "/test",
+  } as PluginInput
+}
+
+function createInput(tool: string): { tool: string; sessionID: string; callID: string } {
+  return {
+    tool,
+    sessionID: "session-1",
+    callID: "call-1",
+  }
+}
+
+describe("createReadImageResizerHook", () => {
+  beforeEach(() => {
+    mockParseImageDimensions.mockReset()
+    mockCalculateTargetDimensions.mockReset()
+    mockResizeImage.mockReset()
+    mockGetSessionModel.mockReset()
+    mockGetSessionModel.mockReturnValue({ providerID: "anthropic", modelID: "claude-sonnet-4-6" })
+  })
+
+  it("skips non-Read tools", async () => {
+    //#given
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "image.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Bash"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("skips when provider is not anthropic", async () => {
+    //#given
+    mockGetSessionModel.mockReturnValue({ providerID: "openai", modelID: "gpt-5.3-codex" })
+    mockParseImageDimensions.mockReturnValue({ width: 3000, height: 2000 })
+    mockCalculateTargetDimensions.mockReturnValue({ width: 1568, height: 1045 })
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "image.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("skips when session model is unknown", async () => {
+    //#given
+    mockGetSessionModel.mockReturnValue(undefined)
+    mockParseImageDimensions.mockReturnValue({ width: 3000, height: 2000 })
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "image.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("skips Read output with no attachments", async () => {
+    //#given
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("skips non-image attachments", async () => {
+    //#given
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "application/pdf", url: "data:application/pdf;base64,AAAA", filename: "file.pdf" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("skips unsupported image mime types", async () => {
+    //#given
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/heic", url: "data:image/heic;base64,AAAA", filename: "photo.heic" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toBe("original output")
+    expect(mockParseImageDimensions).not.toHaveBeenCalled()
+  })
+
+  it("appends within-limits metadata when image is already valid", async () => {
+    //#given
+    mockParseImageDimensions.mockReturnValue({ width: 800, height: 600 })
+    mockCalculateTargetDimensions.mockReturnValue(null)
+
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "image.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toContain("[Image Info]")
+    expect(output.output).toContain("within limits")
+    expect(output.attachments?.[0]?.url).toBe("data:image/png;base64,old")
+    expect(mockResizeImage).not.toHaveBeenCalled()
+  })
+
+  it("replaces attachment URL and appends resize metadata for oversized image", async () => {
+    //#given
+    mockParseImageDimensions.mockReturnValue({ width: 3000, height: 2000 })
+    mockCalculateTargetDimensions.mockReturnValue({ width: 1568, height: 1045 })
+    mockResizeImage.mockResolvedValue({
+      resizedDataUrl: "data:image/png;base64,resized",
+      original: { width: 3000, height: 2000 },
+      resized: { width: 1568, height: 1045 },
+    })
+
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "big.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.attachments?.[0]?.url).toBe("data:image/png;base64,resized")
+    expect(output.output).toContain("[Image Resize Info]")
+    expect(output.output).toContain("resized")
+  })
+
+  it("keeps original attachment URL and marks resize skipped when resize fails", async () => {
+    //#given
+    mockParseImageDimensions.mockReturnValue({ width: 3000, height: 2000 })
+    mockCalculateTargetDimensions.mockReturnValue({ width: 1568, height: 1045 })
+    mockResizeImage.mockResolvedValue(null)
+
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "fail.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.attachments?.[0]?.url).toBe("data:image/png;base64,old")
+    expect(output.output).toContain("resize skipped")
+  })
+
+  it("appends unknown-dimensions metadata when parsing fails", async () => {
+    //#given
+    mockParseImageDimensions.mockReturnValue(null)
+
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "corrupt.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("Read"), output)
+
+    //#then
+    expect(output.output).toContain("dimensions could not be parsed")
+    expect(mockCalculateTargetDimensions).not.toHaveBeenCalled()
+  })
+
+  it("fires for lowercase read tool name", async () => {
+    //#given
+    mockParseImageDimensions.mockReturnValue({ width: 800, height: 600 })
+    mockCalculateTargetDimensions.mockReturnValue(null)
+
+    const hook = createReadImageResizerHook(createMockContext())
+    const output: ToolOutput = {
+      title: "Read",
+      output: "original output",
+      metadata: {},
+      attachments: [{ mime: "image/png", url: "data:image/png;base64,old", filename: "image.png" }],
+    }
+
+    //#when
+    await hook["tool.execute.after"](createInput("read"), output)
+
+    //#then
+    expect(mockParseImageDimensions).toHaveBeenCalledTimes(1)
+    expect(output.output).toContain("within limits")
+  })
+})

src/hooks/read-image-resizer/hook.ts

@@ -0,0 +1,197 @@
+import type { PluginInput } from "@opencode-ai/plugin"
+import type { ImageAttachment, ImageDimensions } from "./types"
+import { parseImageDimensions } from "./image-dimensions"
+import { calculateTargetDimensions, resizeImage } from "./image-resizer"
+import { log } from "../../shared"
+import { getSessionModel } from "../../shared/session-model-state"
+const SUPPORTED_IMAGE_MIMES = new Set(["image/png", "image/jpeg", "image/gif", "image/webp"])
+const TOKEN_DIVISOR = 750
+interface ResizeEntry {
+  filename: string
+  originalDims: ImageDimensions | null
+  resizedDims: ImageDimensions | null
+  status: "resized" | "within-limits" | "resize-skipped" | "unknown-dims"
+}
+function isReadTool(toolName: string): boolean {
+  return toolName.toLowerCase() === "read"
+}
+function asRecord(value: unknown): Record<string, unknown> | null {
+  if (!value || typeof value !== "object" || Array.isArray(value)) {
+    return null
+  }
+  return value as Record<string, unknown>
+}
+function isImageAttachmentRecord(
+  value: Record<string, unknown>,
+): value is Record<string, unknown> & ImageAttachment {
+  const filename = value.filename
+  return (
+    typeof value.mime === "string" &&
+    typeof value.url === "string" &&
+    (typeof filename === "undefined" || typeof filename === "string")
+  )
+}
+function extractImageAttachments(output: Record<string, unknown>): ImageAttachment[] {
+  const attachmentsValue = output.attachments
+  if (!Array.isArray(attachmentsValue)) {
+    return []
+  }
+  const attachments: ImageAttachment[] = []
+  for (const attachmentValue of attachmentsValue) {
+    const attachmentRecord = asRecord(attachmentValue)
+    if (!attachmentRecord) {
+      continue
+    }
+
+    const mime = attachmentRecord.mime
+    const url = attachmentRecord.url
+    if (typeof mime !== "string" || typeof url !== "string") {
+      continue
+    }
+
+    const normalizedMime = mime.toLowerCase()
+    if (!SUPPORTED_IMAGE_MIMES.has(normalizedMime)) {
+      continue
+    }
+
+    attachmentRecord.mime = normalizedMime
+    attachmentRecord.url = url
+    if (isImageAttachmentRecord(attachmentRecord)) {
+      attachments.push(attachmentRecord)
+    }
+  }
+
+  return attachments
+}
+function calculateTokens(width: number, height: number): number {
+  return Math.ceil((width * height) / TOKEN_DIVISOR)
+}
+function formatResizeAppendix(entries: ResizeEntry[]): string {
+  const header = entries.some((entry) => entry.status === "resized") ? "[Image Resize Info]" : "[Image Info]"
+  const lines = [`\n\n${header}`]
+
+  for (const entry of entries) {
+    if (entry.status === "unknown-dims" || !entry.originalDims) {
+      lines.push(`- ${entry.filename}: dimensions could not be parsed`)
+      continue
+    }
+
+    const original = entry.originalDims
+    const originalText = `${original.width}x${original.height}`
+    const originalTokens = calculateTokens(original.width, original.height)
+
+    if (entry.status === "within-limits") {
+      lines.push(`- ${entry.filename}: ${originalText} (within limits, tokens: ${originalTokens})`)
+      continue
+    }
+
+    if (entry.status === "resize-skipped") {
+      lines.push(`- ${entry.filename}: ${originalText} (resize skipped, tokens: ${originalTokens})`)
+      continue
+    }
+
+    if (!entry.resizedDims) {
+      lines.push(`- ${entry.filename}: ${originalText} (resize skipped, tokens: ${originalTokens})`)
+      continue
+    }
+
+    const resized = entry.resizedDims
+    const resizedText = `${resized.width}x${resized.height}`
+    const resizedTokens = calculateTokens(resized.width, resized.height)
+    lines.push(
+      `- ${entry.filename}: ${originalText} -> ${resizedText} (resized, tokens: ${originalTokens} -> ${resizedTokens})`,
+    )
+  }
+
+  return lines.join("\n")
+}
+function resolveFilename(attachment: ImageAttachment, index: number): string {
+  if (attachment.filename && attachment.filename.trim().length > 0) {
+    return attachment.filename
+  }
+
+  return `image-${index + 1}`
+}
+export function createReadImageResizerHook(_ctx: PluginInput) {
+  return {
+    "tool.execute.after": async (
+      input: { tool: string; sessionID: string; callID: string },
+      output: { title: string; output: string; metadata: unknown },
+    ) => {
+      if (!isReadTool(input.tool)) {
+        return
+      }
+
+      const sessionModel = getSessionModel(input.sessionID)
+      if (sessionModel?.providerID !== "anthropic") {
+        return
+      }
+
+      if (typeof output.output !== "string") {
+        return
+      }
+
+      const outputRecord = output as Record<string, unknown>
+      const attachments = extractImageAttachments(outputRecord)
+      if (attachments.length === 0) {
+        return
+      }
+
+      const entries: ResizeEntry[] = []
+      for (const [index, attachment] of attachments.entries()) {
+        const filename = resolveFilename(attachment, index)
+
+        try {
+          const originalDims = parseImageDimensions(attachment.url, attachment.mime)
+          if (!originalDims) {
+            entries.push({ filename, originalDims: null, resizedDims: null, status: "unknown-dims" })
+            continue
+          }
+
+          const targetDims = calculateTargetDimensions(originalDims.width, originalDims.height)
+          if (!targetDims) {
+            entries.push({
+              filename,
+              originalDims,
+              resizedDims: null,
+              status: "within-limits",
+            })
+            continue
+          }
+
+          const resizedResult = await resizeImage(attachment.url, attachment.mime, targetDims)
+          if (!resizedResult) {
+            entries.push({
+              filename,
+              originalDims,
+              resizedDims: null,
+              status: "resize-skipped",
+            })
+            continue
+          }
+
+          attachment.url = resizedResult.resizedDataUrl
+
+          entries.push({
+            filename,
+            originalDims: resizedResult.original,
+            resizedDims: resizedResult.resized,
+            status: "resized",
+          })
+        } catch (error) {
+          log("[read-image-resizer] attachment processing failed", {
+            error: error instanceof Error ? error.message : String(error),
+            filename,
+          })
+          entries.push({ filename, originalDims: null, resizedDims: null, status: "unknown-dims" })
+        }
+      }
+
+      if (entries.length === 0) {
+        return
+      }
+
+      output.output += formatResizeAppendix(entries)
+    },
+  }
+}

src/hooks/read-image-resizer/image-dimensions.test.ts

@@ -0,0 +1,108 @@
+/// <reference types="bun-types" />
+
+import { describe, expect, it } from "bun:test"
+
+import { parseImageDimensions } from "./image-dimensions"
+
+const PNG_1X1_DATA_URL =
+  "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
+
+const GIF_1X1_DATA_URL =
+  "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7"
+
+function createPngDataUrl(width: number, height: number): string {
+  const buf = Buffer.alloc(33)
+  buf.set([0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a], 0)
+  buf.writeUInt32BE(13, 8)
+  buf.set([0x49, 0x48, 0x44, 0x52], 12)
+  buf.writeUInt32BE(width, 16)
+  buf.writeUInt32BE(height, 20)
+  return `data:image/png;base64,${buf.toString("base64")}`
+}
+
+function createGifDataUrl(width: number, height: number): string {
+  const buf = Buffer.alloc(10)
+  buf.set([0x47, 0x49, 0x46, 0x38, 0x39, 0x61], 0)
+  buf.writeUInt16LE(width, 6)
+  buf.writeUInt16LE(height, 8)
+  return `data:image/gif;base64,${buf.toString("base64")}`
+}
+
+describe("parseImageDimensions", () => {
+  it("parses PNG 1x1 dimensions", () => {
+    //#given
+    const dataUrl = PNG_1X1_DATA_URL
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/png")
+
+    //#then
+    expect(result).toEqual({ width: 1, height: 1 })
+  })
+
+  it("parses PNG dimensions from IHDR", () => {
+    //#given
+    const dataUrl = createPngDataUrl(3000, 2000)
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/png")
+
+    //#then
+    expect(result).toEqual({ width: 3000, height: 2000 })
+  })
+
+  it("parses GIF 1x1 dimensions", () => {
+    //#given
+    const dataUrl = GIF_1X1_DATA_URL
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/gif")
+
+    //#then
+    expect(result).toEqual({ width: 1, height: 1 })
+  })
+
+  it("parses GIF dimensions from logical screen descriptor", () => {
+    //#given
+    const dataUrl = createGifDataUrl(320, 240)
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/gif")
+
+    //#then
+    expect(result).toEqual({ width: 320, height: 240 })
+  })
+
+  it("returns null for empty input", () => {
+    //#given
+    const dataUrl = ""
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/png")
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null for too-short PNG buffer", () => {
+    //#given
+    const dataUrl = "data:image/png;base64,AAAA"
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/png")
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null for unsupported mime type", () => {
+    //#given
+    const dataUrl = PNG_1X1_DATA_URL
+
+    //#when
+    const result = parseImageDimensions(dataUrl, "image/heic")
+
+    //#then
+    expect(result).toBeNull()
+  })
+})

src/hooks/read-image-resizer/image-dimensions.ts

@@ -0,0 +1,187 @@
+import type { ImageDimensions } from "./types"
+
+import { extractBase64Data } from "../../tools/look-at/mime-type-inference"
+
+function toImageDimensions(width: number, height: number): ImageDimensions | null {
+  if (!Number.isFinite(width) || !Number.isFinite(height)) {
+    return null
+  }
+
+  if (width <= 0 || height <= 0) {
+    return null
+  }
+
+  return { width, height }
+}
+
+function parsePngDimensions(buffer: Buffer): ImageDimensions | null {
+  if (buffer.length < 24) {
+    return null
+  }
+
+  const isPngSignature =
+    buffer[0] === 0x89 &&
+    buffer[1] === 0x50 &&
+    buffer[2] === 0x4e &&
+    buffer[3] === 0x47 &&
+    buffer[4] === 0x0d &&
+    buffer[5] === 0x0a &&
+    buffer[6] === 0x1a &&
+    buffer[7] === 0x0a
+
+  if (!isPngSignature || buffer.toString("ascii", 12, 16) !== "IHDR") {
+    return null
+  }
+
+  const width = buffer.readUInt32BE(16)
+  const height = buffer.readUInt32BE(20)
+  return toImageDimensions(width, height)
+}
+
+function parseGifDimensions(buffer: Buffer): ImageDimensions | null {
+  if (buffer.length < 10) {
+    return null
+  }
+
+  if (buffer.toString("ascii", 0, 4) !== "GIF8") {
+    return null
+  }
+
+  const width = buffer.readUInt16LE(6)
+  const height = buffer.readUInt16LE(8)
+  return toImageDimensions(width, height)
+}
+
+function parseJpegDimensions(buffer: Buffer): ImageDimensions | null {
+  if (buffer.length < 4 || buffer[0] !== 0xff || buffer[1] !== 0xd8) {
+    return null
+  }
+
+  let offset = 2
+
+  while (offset < buffer.length) {
+    if (buffer[offset] !== 0xff) {
+      offset += 1
+      continue
+    }
+
+    while (offset < buffer.length && buffer[offset] === 0xff) {
+      offset += 1
+    }
+
+    if (offset >= buffer.length) {
+      return null
+    }
+
+    const marker = buffer[offset]
+    offset += 1
+
+    if (marker === 0xd9 || marker === 0xda) {
+      break
+    }
+
+    if (offset + 1 >= buffer.length) {
+      return null
+    }
+
+    const segmentLength = buffer.readUInt16BE(offset)
+    if (segmentLength < 2) {
+      return null
+    }
+
+    if ((marker === 0xc0 || marker === 0xc2) && offset + 7 < buffer.length) {
+      const height = buffer.readUInt16BE(offset + 3)
+      const width = buffer.readUInt16BE(offset + 5)
+      return toImageDimensions(width, height)
+    }
+
+    offset += segmentLength
+  }
+
+  return null
+}
+
+function readUInt24LE(buffer: Buffer, offset: number): number {
+  return buffer[offset] | (buffer[offset + 1] << 8) | (buffer[offset + 2] << 16)
+}
+
+function parseWebpDimensions(buffer: Buffer): ImageDimensions | null {
+  if (buffer.length < 16) {
+    return null
+  }
+
+  if (buffer.toString("ascii", 0, 4) !== "RIFF" || buffer.toString("ascii", 8, 12) !== "WEBP") {
+    return null
+  }
+
+  const chunkType = buffer.toString("ascii", 12, 16)
+
+  if (chunkType === "VP8 ") {
+    if (buffer[23] !== 0x9d || buffer[24] !== 0x01 || buffer[25] !== 0x2a) {
+      return null
+    }
+
+    const width = buffer.readUInt16LE(26) & 0x3fff
+    const height = buffer.readUInt16LE(28) & 0x3fff
+    return toImageDimensions(width, height)
+  }
+
+  if (chunkType === "VP8L") {
+    if (buffer.length < 25 || buffer[20] !== 0x2f) {
+      return null
+    }
+
+    const bits = buffer.readUInt32LE(21)
+    const width = (bits & 0x3fff) + 1
+    const height = ((bits >>> 14) & 0x3fff) + 1
+    return toImageDimensions(width, height)
+  }
+
+  if (chunkType === "VP8X") {
+    const width = readUInt24LE(buffer, 24) + 1
+    const height = readUInt24LE(buffer, 27) + 1
+    return toImageDimensions(width, height)
+  }
+
+  return null
+}
+
+export function parseImageDimensions(base64DataUrl: string, mimeType: string): ImageDimensions | null {
+  try {
+    if (!base64DataUrl || !mimeType) {
+      return null
+    }
+
+    const rawBase64 = extractBase64Data(base64DataUrl)
+    if (!rawBase64) {
+      return null
+    }
+
+    const buffer = Buffer.from(rawBase64, "base64")
+    if (buffer.length === 0) {
+      return null
+    }
+
+    const normalizedMime = mimeType.toLowerCase()
+
+    if (normalizedMime === "image/png") {
+      return parsePngDimensions(buffer)
+    }
+
+    if (normalizedMime === "image/gif") {
+      return parseGifDimensions(buffer)
+    }
+
+    if (normalizedMime === "image/jpeg" || normalizedMime === "image/jpg") {
+      return parseJpegDimensions(buffer)
+    }
+
+    if (normalizedMime === "image/webp") {
+      return parseWebpDimensions(buffer)
+    }
+
+    return null
+  } catch {
+    return null
+  }
+}

src/hooks/read-image-resizer/image-resizer.test.ts

@@ -0,0 +1,132 @@
+/// <reference types="bun-types" />
+
+import { afterEach, describe, expect, it, mock } from "bun:test"
+
+const PNG_1X1_DATA_URL =
+  "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
+
+type ImageResizerModule = typeof import("./image-resizer")
+
+async function importFreshImageResizerModule(): Promise<ImageResizerModule> {
+  return import(`./image-resizer?test-${Date.now()}-${Math.random()}`)
+}
+
+describe("calculateTargetDimensions", () => {
+  it("returns null when dimensions are already within limits", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(800, 600)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null at exact long-edge boundary", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(1568, 1000)
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("scales landscape dimensions by max long edge", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(3000, 2000)
+
+    //#then
+    expect(result).toEqual({
+      width: 1568,
+      height: Math.floor(2000 * (1568 / 3000)),
+    })
+  })
+
+  it("scales portrait dimensions by max long edge", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(2000, 3000)
+
+    //#then
+    expect(result).toEqual({
+      width: Math.floor(2000 * (1568 / 3000)),
+      height: 1568,
+    })
+  })
+
+  it("scales square dimensions to exact target", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(4000, 4000)
+
+    //#then
+    expect(result).toEqual({ width: 1568, height: 1568 })
+  })
+
+  it("uses custom maxLongEdge when provided", async () => {
+    //#given
+    const { calculateTargetDimensions } = await importFreshImageResizerModule()
+
+    //#when
+    const result = calculateTargetDimensions(2000, 1000, 1000)
+
+    //#then
+    expect(result).toEqual({ width: 1000, height: 500 })
+  })
+})
+
+describe("resizeImage", () => {
+  afterEach(() => {
+    mock.restore()
+  })
+
+  it("returns null when sharp import fails", async () => {
+    //#given
+    mock.module("sharp", () => {
+      throw new Error("sharp unavailable")
+    })
+    const { resizeImage } = await importFreshImageResizerModule()
+
+    //#when
+    const result = await resizeImage(PNG_1X1_DATA_URL, "image/png", {
+      width: 1,
+      height: 1,
+    })
+
+    //#then
+    expect(result).toBeNull()
+  })
+
+  it("returns null when sharp throws during resize", async () => {
+    //#given
+    const mockSharpFactory = mock(() => ({
+      resize: () => {
+        throw new Error("resize failed")
+      },
+    }))
+
+    mock.module("sharp", () => ({
+      default: mockSharpFactory,
+    }))
+    const { resizeImage } = await importFreshImageResizerModule()
+
+    //#when
+    const result = await resizeImage(PNG_1X1_DATA_URL, "image/png", {
+      width: 1,
+      height: 1,
+    })
+
+    //#then
+    expect(result).toBeNull()
+  })
+})

src/hooks/read-image-resizer/image-resizer.ts

@@ -0,0 +1,184 @@
+import type { ImageDimensions, ResizeResult } from "./types"
+import { extractBase64Data } from "../../tools/look-at/mime-type-inference"
+import { log } from "../../shared"
+
+const ANTHROPIC_MAX_LONG_EDGE = 1568
+const ANTHROPIC_MAX_FILE_SIZE = 5 * 1024 * 1024
+
+type SharpFormat = "jpeg" | "png" | "gif" | "webp"
+
+interface SharpMetadata {
+  width?: number
+  height?: number
+}
+
+interface SharpInstance {
+  resize(width: number, height: number, options: { fit: "inside" }): SharpInstance
+  toFormat(format: SharpFormat, options?: { quality?: number }): SharpInstance
+  toBuffer(): Promise<Buffer>
+  metadata(): Promise<SharpMetadata>
+}
+
+type SharpFactory = (input: Buffer) => SharpInstance
+
+function resolveSharpFactory(sharpModule: unknown): SharpFactory | null {
+  if (typeof sharpModule === "function") {
+    return sharpModule as SharpFactory
+  }
+
+  if (!sharpModule || typeof sharpModule !== "object") {
+    return null
+  }
+
+  const defaultExport = Reflect.get(sharpModule, "default")
+  return typeof defaultExport === "function" ? (defaultExport as SharpFactory) : null
+}
+
+function resolveSharpFormat(mimeType: string): SharpFormat {
+  const normalizedMime = mimeType.toLowerCase()
+  if (normalizedMime === "image/png") {
+    return "png"
+  }
+  if (normalizedMime === "image/gif") {
+    return "gif"
+  }
+  if (normalizedMime === "image/webp") {
+    return "webp"
+  }
+  return "jpeg"
+}
+
+function canAdjustQuality(format: SharpFormat): boolean {
+  return format === "jpeg" || format === "webp"
+}
+
+function toDimensions(metadata: SharpMetadata): ImageDimensions | null {
+  const { width, height } = metadata
+  if (!width || !height) {
+    return null
+  }
+  return { width, height }
+}
+
+async function renderResizedBuffer(args: {
+  sharpFactory: SharpFactory
+  inputBuffer: Buffer
+  target: ImageDimensions
+  format: SharpFormat
+  quality?: number
+}): Promise<Buffer> {
+  const { sharpFactory, inputBuffer, target, format, quality } = args
+  return sharpFactory(inputBuffer)
+    .resize(target.width, target.height, { fit: "inside" })
+    .toFormat(format, quality ? { quality } : undefined)
+    .toBuffer()
+}
+
+function getErrorMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error)
+}
+
+export function calculateTargetDimensions(
+  width: number,
+  height: number,
+  maxLongEdge = ANTHROPIC_MAX_LONG_EDGE,
+): ImageDimensions | null {
+  if (width <= 0 || height <= 0 || maxLongEdge <= 0) {
+    return null
+  }
+
+  const longEdge = Math.max(width, height)
+  if (longEdge <= maxLongEdge) {
+    return null
+  }
+
+  if (width >= height) {
+    return {
+      width: maxLongEdge,
+      height: Math.max(1, Math.floor((height * maxLongEdge) / width)),
+    }
+  }
+
+  return {
+    width: Math.max(1, Math.floor((width * maxLongEdge) / height)),
+    height: maxLongEdge,
+  }
+}
+
+export async function resizeImage(
+  base64DataUrl: string,
+  mimeType: string,
+  target: ImageDimensions,
+): Promise<ResizeResult | null> {
+  try {
+    const sharpModuleName = "sharp"
+    const sharpModule = await import(sharpModuleName).catch(() => null)
+    if (!sharpModule) {
+      log("[read-image-resizer] sharp unavailable, skipping resize")
+      return null
+    }
+
+    const sharpFactory = resolveSharpFactory(sharpModule)
+    if (!sharpFactory) {
+      log("[read-image-resizer] sharp import has unexpected shape")
+      return null
+    }
+
+    const rawBase64 = extractBase64Data(base64DataUrl)
+    if (!rawBase64) {
+      return null
+    }
+
+    const inputBuffer = Buffer.from(rawBase64, "base64")
+    if (inputBuffer.length === 0) {
+      return null
+    }
+
+    const original = toDimensions(await sharpFactory(inputBuffer).metadata())
+    if (!original) {
+      return null
+    }
+
+    const format = resolveSharpFormat(mimeType)
+    let resizedBuffer = await renderResizedBuffer({
+      sharpFactory,
+      inputBuffer,
+      target,
+      format,
+    })
+
+    if (resizedBuffer.length > ANTHROPIC_MAX_FILE_SIZE && canAdjustQuality(format)) {
+      for (const quality of [80, 60, 40]) {
+        resizedBuffer = await renderResizedBuffer({
+          sharpFactory,
+          inputBuffer,
+          target,
+          format,
+          quality,
+        })
+
+        if (resizedBuffer.length <= ANTHROPIC_MAX_FILE_SIZE) {
+          break
+        }
+      }
+    }
+
+    const resized = toDimensions(await sharpFactory(resizedBuffer).metadata())
+    if (!resized) {
+      return null
+    }
+
+    return {
+      resizedDataUrl: `data:${mimeType};base64,${resizedBuffer.toString("base64")}`,
+      original,
+      resized,
+    }
+  } catch (error) {
+    log("[read-image-resizer] resize failed", {
+      error: getErrorMessage(error),
+      mimeType,
+      target,
+    })
+    return null
+  }
+}

src/hooks/read-image-resizer/index.ts

@@ -0,0 +1 @@
+export { createReadImageResizerHook } from "./hook"

src/hooks/read-image-resizer/types.ts

@@ -0,0 +1,16 @@
+export interface ImageDimensions {
+  width: number
+  height: number
+}
+
+export interface ImageAttachment {
+  mime: string
+  url: string
+  filename?: string
+}
+
+export interface ResizeResult {
+  resizedDataUrl: string
+  original: ImageDimensions
+  resized: ImageDimensions
+}

src/plugin/hooks/create-tool-guard-hooks.ts

@@ -12,6 +12,7 @@ import {
   createTasksTodowriteDisablerHook,
   createWriteExistingFileGuardHook,
   createHashlineReadEnhancerHook,
+  createReadImageResizerHook,
   createJsonErrorRecoveryHook,
 } from "../../hooks"
 import {
@@ -33,6 +34,7 @@ export type ToolGuardHooks = {
   writeExistingFileGuard: ReturnType<typeof createWriteExistingFileGuardHook> | null
   hashlineReadEnhancer: ReturnType<typeof createHashlineReadEnhancerHook> | null
   jsonErrorRecovery: ReturnType<typeof createJsonErrorRecoveryHook> | null
+  readImageResizer: ReturnType<typeof createReadImageResizerHook> | null
 }
 
 export function createToolGuardHooks(args: {
@@ -105,6 +107,10 @@ export function createToolGuardHooks(args: {
     ? safeHook("json-error-recovery", () => createJsonErrorRecoveryHook(ctx))
     : null
 
+  const readImageResizer = isHookEnabled("read-image-resizer")
+    ? safeHook("read-image-resizer", () => createReadImageResizerHook(ctx))
+    : null
+
   return {
     commentChecker,
     toolOutputTruncator,
@@ -116,5 +122,6 @@ export function createToolGuardHooks(args: {
     writeExistingFileGuard,
     hashlineReadEnhancer,
     jsonErrorRecovery,
+    readImageResizer,
   }
 }

src/plugin/tool-execute-after.ts

@@ -43,6 +43,7 @@ export function createToolExecuteAfterHandler(args: {
     await hooks.delegateTaskRetry?.["tool.execute.after"]?.(input, output)
     await hooks.atlasHook?.["tool.execute.after"]?.(input, output)
     await hooks.taskResumeInfo?.["tool.execute.after"]?.(input, output)
+    await hooks.readImageResizer?.["tool.execute.after"]?.(input, output)
     await hooks.hashlineReadEnhancer?.["tool.execute.after"]?.(input, output)
     await hooks.jsonErrorRecovery?.["tool.execute.after"]?.(input, output)
   }