diff --git a/packages/core/src/util/error.ts b/packages/core/src/util/error.ts
index 5fe41e1cff99..a1caa323e5ec 100644
--- a/packages/core/src/util/error.ts
+++ b/packages/core/src/util/error.ts
@@ -40,7 +40,13 @@ export abstract class NamedError extends Error {
         public readonly data: Data,
         options?: ErrorOptions,
       ) {
-        super(name, options)
+        // Error.message is the human detail only; the tag lives on `.name`, and renderers
+        // (Cause.pretty, Error.toString) compose "${name}: ${message}".
+        const message =
+          typeof data === "object" && data !== null && "message" in data && typeof data.message === "string"
+            ? data.message
+            : ""
+        super(message, options)
         this.name = name
       }
 
diff --git a/packages/core/src/util/log.ts b/packages/core/src/util/log.ts
index c395ac017552..43d6cc4a4375 100644
--- a/packages/core/src/util/log.ts
+++ b/packages/core/src/util/log.ts
@@ -106,7 +106,9 @@ async function cleanup(dir: string) {
 }
 
 function formatError(error: Error, depth = 0): string {
-  const result = error.message
+  // toString() composes "${name}: ${message}" (and drops the colon when message is empty),
+  // so the error's tag is never lost the way a bare error.message would lose it.
+  const result = error.toString()
   return error.cause instanceof Error && depth < 10
     ? result + " Caused by: " + formatError(error.cause, depth + 1)
     : result
diff --git a/packages/core/test/util-error.test.ts b/packages/core/test/util-error.test.ts
new file mode 100644
index 000000000000..73fff259d1c1
--- /dev/null
+++ b/packages/core/test/util-error.test.ts
@@ -0,0 +1,37 @@
+import { describe, expect, it } from "bun:test"
+import { Cause, Schema } from "effect"
+import { NamedError } from "@opencode-ai/core/util/error"
+
+const WithMessage = NamedError.create("FooError", { message: Schema.String })
+const NoMessage = NamedError.create("BarError", { providerID: Schema.String })
+
+const prettyHead = (error: Error) => Cause.pretty(Cause.fail(error)).split("\n")[0]
+
+describe("NamedError rendering", () => {
+  it("puts the human detail on .message and the tag on .name", () => {
+    const error = new WithMessage({ message: "boom" })
+    expect(error.name).toBe("FooError")
+    expect(error.message).toBe("boom")
+  })
+
+  it("composes 'name: message' exactly once, with no doubled tag", () => {
+    const error = new WithMessage({ message: "boom" })
+    expect(error.toString()).toBe("FooError: boom")
+    expect(prettyHead(error)).toBe("FooError: boom")
+  })
+
+  it("renders only the tag when data has no message field", () => {
+    const error = new NoMessage({ providerID: "anthropic" })
+    expect(error.message).toBe("")
+    expect(error.toString()).toBe("BarError")
+    // Cause.pretty composes "${name}: ${message}" unconditionally, so an empty message
+    // leaves a trailing ": " here. Acceptable; the log path uses toString(), which omits it.
+    expect(prettyHead(error)).toBe("BarError: ")
+  })
+
+  it("ignores a non-string message field", () => {
+    const error = new WithMessage({ message: undefined as unknown as string })
+    expect(error.message).toBe("")
+    expect(error.toString()).toBe("FooError")
+  })
+})