anomalyco · opencode-agent · Jun 5, 2026
diff --git a/packages/core/src/filesystem.ts b/packages/core/src/filesystem.ts
@@ -25,6 +25,28 @@ export const MAX_READ_BYTES = 50 * 1024
 const MAX_LINE_LENGTH = 2_000
 const MAX_LINE_SUFFIX = `... (line truncated to ${MAX_LINE_LENGTH} chars)`
 
+export class ReadLimitError extends Error {
+  readonly resource: string
+  readonly maximumBytes: number
+
+  constructor(resource: string, maximumBytes: number) {
+    super(`File exceeds ${maximumBytes} byte read limit: ${resource}`)
+    this.name = "ReadLimitError"
+    this.resource = resource
+    this.maximumBytes = maximumBytes
+  }
+}
+
+export class BinaryFileError extends Error {
+  readonly resource: string
+
+  constructor(resource: string) {
+    super(`Cannot read binary file: ${resource}`)
+    this.name = "BinaryFileError"
+    this.resource = resource
+  }
+}
+
 export class TextContent extends Schema.Class<TextContent>("FileSystem.TextContent")({
   type: Schema.Literal("text"),
   content: Schema.String,
@@ -315,11 +337,10 @@ export const layer = Layer.effect(
           if (info.type !== "File") return yield* Effect.die(new Error("Path is not a file"))
           if (info.dev !== target.dev || Option.getOrUndefined(info.ino) !== target.ino)
             return yield* Effect.die(new Error("File changed after permission approval"))
-          if (info.size > maximumBytes)
-            return yield* Effect.die(new Error(`File exceeds ${maximumBytes} byte read limit`))
+          if (info.size > maximumBytes) return yield* Effect.die(new ReadLimitError(target.resource, maximumBytes))
           const bytes = yield* file.readAlloc(maximumBytes + 1).pipe(Effect.orDie)
           if (bytes._tag === "Some" && bytes.value.length > maximumBytes)
-            return yield* Effect.die(new Error(`File exceeds ${maximumBytes} byte read limit`))
+            return yield* Effect.die(new ReadLimitError(target.resource, maximumBytes))
           return yield* content(target, bytes._tag === "Some" ? bytes.value : new Uint8Array())
         }),
       )
@@ -376,7 +397,7 @@ export const layer = Layer.effect(
           while (!done) {
             const chunk = yield* file.readAlloc(64 * 1024).pipe(Effect.orDie)
             if (Option.isNone(chunk)) break
-            if (chunk.value.includes(0)) return yield* Effect.die(new Error("Cannot page binary file"))
+            if (chunk.value.includes(0)) return yield* Effect.die(new BinaryFileError(target.resource))
             let text = decoder.decode(chunk.value, { stream: true })
             while (true) {
               const index = text.indexOf("\n")

diff --git a/packages/core/src/tool/read.ts b/packages/core/src/tool/read.ts
@@ -79,12 +79,14 @@ export const layer = Layer.effectDiscard(
             return yield* filesystem.readResolved(final.target, FileSystem.MAX_READ_BYTES)
           }).pipe(
             Effect.catchCause((cause) =>
-              Effect.fail(
-                new ToolFailure({
-                  message: `Unable to read ${"resource" in input ? input.resource : input.path}`,
-                  error: Cause.squash(cause),
-                }),
-              ),
+              Effect.gen(function* () {
+                const error = Cause.squash(cause)
+                const message =
+                  error instanceof FileSystem.BinaryFileError || error instanceof FileSystem.ReadLimitError
+                    ? error.message
+                    : `Unable to read ${"resource" in input ? input.resource : input.path}`
+                return yield* new ToolFailure({ message, error })
+              }),
             ),
           )
         },

diff --git a/packages/core/test/tool-read.test.ts b/packages/core/test/tool-read.test.ts
@@ -21,6 +21,7 @@ let listReal = "/project/src"
 let size = 5
 let real = "/project/README.md"
 let afterApproval = () => {}
+let readFailure: unknown
 const resourceReads: ToolOutputStore.ReadInput[] = []
 const filesystem = Layer.succeed(
   FileSystem.Service,
@@ -67,22 +68,26 @@ const filesystem = Layer.succeed(
         ),
       ),
     readResolved: () =>
-      Effect.sync(() => {
-        reads.push({ path: RelativePath.make("README.md") })
-        return new FileSystem.TextContent({ type: "text", content: "hello", mime: "text/plain" })
-      }),
+      readFailure === undefined
+        ? Effect.sync(() => {
+            reads.push({ path: RelativePath.make("README.md") })
+            return new FileSystem.TextContent({ type: "text", content: "hello", mime: "text/plain" })
+          })
+        : Effect.die(readFailure),
     readTextPageResolved: (_target, page = {}) =>
-      Effect.sync(() => {
-        textPageInputs.push(page)
-        return new FileSystem.TextPage({
-          type: "text-page",
-          content: "hello",
-          mime: "text/plain",
-          offset: page.offset ?? 1,
-          truncated: true,
-          next: (page.offset ?? 1) + 1,
-        })
-      }),
+      readFailure === undefined
+        ? Effect.sync(() => {
+            textPageInputs.push(page)
+            return new FileSystem.TextPage({
+              type: "text-page",
+              content: "hello",
+              mime: "text/plain",
+              offset: page.offset ?? 1,
+              truncated: true,
+              next: (page.offset ?? 1) + 1,
+            })
+          })
+        : Effect.die(readFailure),
     resolveRoot: () => Effect.die("unused"),
     revalidateRoot: Effect.succeed,
     list: () => Effect.die("unused"),
@@ -167,6 +172,7 @@ describe("ReadTool", () => {
       size = 5
       real = "/project/README.md"
       afterApproval = () => {}
+      readFailure = undefined
       resolvedInput = undefined
       const registry = yield* ToolRegistry.Service
 
@@ -357,6 +363,7 @@ describe("ReadTool", () => {
       size = FileSystem.MAX_READ_BYTES + 1
       real = "/project/large.txt"
       afterApproval = () => {}
+      readFailure = undefined
       const registry = yield* ToolRegistry.Service
 
       expect(
@@ -377,6 +384,26 @@ describe("ReadTool", () => {
     }),
   )
 
+  it.effect("reports the binary file that cannot be paged", () =>
+    Effect.gen(function* () {
+      allow = true
+      resolveFailure = undefined
+      listResolveFailure = new Error("not a directory")
+      size = FileSystem.MAX_READ_BYTES + 1
+      real = "/project/archive.zip"
+      afterApproval = () => {}
+      readFailure = new FileSystem.BinaryFileError("archive.zip")
+      const registry = yield* ToolRegistry.Service
+
+      expect(
+        yield* registry.execute({
+          sessionID,
+          call: { type: "tool-call", id: "call-binary", name: "read", input: { path: "archive.zip" } },
+        }),
+      ).toEqual({ type: "error", value: "Cannot read binary file: archive.zip" })
+    }),
+  )
+
   it.effect("does not read when the file changes after permission approval", () =>
     Effect.gen(function* () {
       assertions.length = 0