diff --git a/src/api/error-handler.ts b/src/api/error-handler.ts
new file mode 100644
index 000000000..c60befed5
--- /dev/null
+++ b/src/api/error-handler.ts
@@ -0,0 +1,104 @@
+// The app's global error boundary (`app.onError`).
+//
+// ── WHAT THIS DOES AND DOES NOT CHANGE ───────────────────────────────────────────────────────────────
+// Hono ALWAYS installs a default error handler internally (hono-base.js), so an uncaught route throw was
+// never an unhandled crash: it already became a 500, and `compose.js` sets `context.error` BEFORE dispatching
+// to whichever handler is installed — which is what worker-posthog.ts's capture middleware reads. Installing
+// this one preserves that (the assignment is unconditional and upstream of the handler), so error capture is
+// unaffected. What the default gets WRONG for this app is the shape of what it returns and logs:
+//
+//   1. `c.text("Internal Server Error", 500)` — a text/plain body from a JSON API. Every other route in this
+//      app answers `{ error: ... }`, so a client parsing JSON hits a parse failure on precisely the response
+//      it most needs to understand. Now a 500 is `{ error: "internal_error" }` like every other error here.
+//
+//   2. `console.error(err)` — a raw Error object. This codebase's log forwarder keys on STRUCTURED JSON
+//      (`{level:"error", event, ...}`); a bare Error is not classified, and carries no method/path, so a
+//      production 500 could not be traced back to the route that raised it. Now it is one structured line
+//      with the request's method and path.
+//
+// ── WHAT IS DELIBERATELY PRESERVED ───────────────────────────────────────────────────────────────────
+// An `HTTPException` carries its OWN response and status. Hono's default returns it verbatim, and so does
+// this: swallowing it into a 500 would turn every intentional 401/403/404/413/422 raised by middleware into
+// an opaque server error, which is the single most damaging thing a global handler can get wrong.
+//
+// ── WHAT NEVER REACHES THE CLIENT ────────────────────────────────────────────────────────────────────
+// The body carries a fixed code and nothing else. No message, no stack, no cause — several routes here are
+// unauthenticated, and an error string can carry a query fragment, a binding name, or an upstream URL. The
+// detail goes to the log, which is operator-only, and is bounded there.
+import type { Context, MiddlewareHandler } from "hono";
+import { errorMessage, errorStack } from "../utils/json";
+
+/** The single body every unexpected failure returns. A stable code rather than prose: a client can branch on
+ *  it, and it cannot accidentally grow to include something internal. */
+export const INTERNAL_ERROR_BODY = { error: "internal_error" } as const;
+
+/**
+ * The global `onError` handler. Exported so it can be tested through a real Hono dispatch rather than by
+ * faking a Context — the failure modes worth pinning (status preservation, no leakage) are only meaningful
+ * end to end.
+ */
+export function handleAppError(error: Error, c: Context): Response {
+  // Duck-typed on `getResponse` exactly as Hono's own default handler does, rather than
+  // `instanceof HTTPException`: a workspace with several packages depending on hono can end up with two
+  // copies in one module graph, and an `instanceof` check across them would collapse an intentional 4xx
+  // into a 500. Asserted with a hand-rolled foreign exception in the tests.
+  if (typeof error === "object" && "getResponse" in error && typeof (error as { getResponse: unknown }).getResponse === "function") {
+    const response = (error as unknown as { getResponse: () => Response }).getResponse();
+    // `c.newResponse(res.body, res)` rather than returning `res` directly, mirroring Hono's default: it
+    // re-applies the context's own headers (CORS, content-type negotiation) that middleware already set.
+    return c.newResponse(response.body, response);
+  }
+  console.error(
+    JSON.stringify({
+      level: "error",
+      event: "unhandled_route_error",
+      method: c.req.method,
+      // `c.req.path` is the matched path WITHOUT the query string, so a token or id in a query parameter
+      // cannot ride into the log.
+      path: c.req.path,
+      message: errorMessage(error),
+      stack: errorStack(error),
+    }),
+  );
+  return c.json(INTERNAL_ERROR_BODY, 500);
+}
+
+/**
+ * The other half of the boundary: a thrown NON-Error.
+ *
+ * Hono's `compose()` routes a caught throw to `onError` only when it is `instanceof Error` -- otherwise it
+ * RE-THROWS (`else { throw err; }`). So `throw "a string"`, a rejected promise carrying a plain object, or a
+ * library that throws a non-Error escapes `app.onError` entirely: no JSON response, no structured log, and
+ * no `context.error`, so the PostHog capture middleware never sees it either. Verified empirically, not
+ * assumed -- a test throws a bare string and, without this, the request rejects rather than responding.
+ *
+ * A try/catch around `next()` is the right shape for exactly this case and ONLY this case: an `Error` thrown
+ * below has already been converted to a response several dispatch levels down (which is why
+ * worker-posthog.ts documents that a try/catch here would never see one), so this adds no second handling
+ * path for the normal case -- it catches precisely what `onError` structurally cannot.
+ *
+ * Register OUTERMOST so it also covers a non-Error thrown by another middleware.
+ */
+export function nonErrorBoundary(): MiddlewareHandler {
+  return async (c, next) => {
+    try {
+      await next();
+    } catch (thrown) {
+      // An Error reaching here would mean Hono's own handling changed; re-throw rather than silently
+      // duplicating onError's job, so that change surfaces instead of being masked.
+      if (thrown instanceof Error) throw thrown;
+      console.error(
+        JSON.stringify({
+          level: "error",
+          event: "unhandled_non_error_throw",
+          method: c.req.method,
+          path: c.req.path,
+          // `String(thrown)` rather than the value: a thrown object could serialize to something enormous
+          // or circular, and this line has to be loggable no matter what was thrown.
+          thrown: String(thrown).slice(0, 500),
+        }),
+      );
+      return c.json(INTERNAL_ERROR_BODY, 500);
+    }
+  };
+}
diff --git a/src/api/routes.ts b/src/api/routes.ts
index 6e6b6bca9..96ded3b59 100644
--- a/src/api/routes.ts
+++ b/src/api/routes.ts
@@ -1,5 +1,6 @@
 import { Hono, type Context } from "hono";
 import { requiresApiToken } from "../auth/route-auth";
+import { handleAppError, nonErrorBoundary } from "./error-handler";
 import { createWorkerPostHogErrorMiddleware } from "./worker-posthog";
 import { z } from "zod";
 import { parsePositiveInt } from "../utils/json";
@@ -1162,6 +1163,13 @@ function internalOpsAgentConfig(env: Env): OpsAgentConfig {
 
 export function createApp() {
   const app = new Hono<AppBindings>();
+  // The global error boundary. Hono installs its own default regardless, so this REPLACES a handler that
+  // returned a text/plain body from a JSON API and logged a raw Error the forwarder cannot classify --
+  // see error-handler.ts for what it preserves (HTTPException status) and what it refuses to leak.
+  app.onError(handleAppError);
+  // Registered OUTERMOST: Hono routes only `instanceof Error` to onError and RE-THROWS anything else, so a
+  // thrown non-Error would otherwise escape the boundary entirely -- no response, no log, no c.error.
+  app.use("*", nonErrorBoundary());
   // Registered FIRST/outermost so it wraps every other middleware and route below, including a thrown
   // exception from the CORS/rate-limit middleware right after this. REPLACES the old Sentry middleware
   // entirely (2026-07-25 epic #8286 correction: full replacement, not a parallel-run). No-ops completely
diff --git a/test/unit/api-error-handler.test.ts b/test/unit/api-error-handler.test.ts
new file mode 100644
index 000000000..379396fe9
--- /dev/null
+++ b/test/unit/api-error-handler.test.ts
@@ -0,0 +1,218 @@
+import { describe, expect, it, vi } from "vitest";
+import { Hono } from "hono";
+import { HTTPException } from "hono/http-exception";
+import { handleAppError, nonErrorBoundary, INTERNAL_ERROR_BODY } from "../../src/api/error-handler";
+
+// The global error boundary. Every assertion below runs through a REAL Hono dispatch rather than a faked
+// Context: the properties worth pinning (status preservation, no leakage, `c.error` still being set for the
+// PostHog capture) are only meaningful end to end, and a hand-built context would let all three pass while
+// the wiring was wrong.
+
+/** A minimal app wired exactly the way createApp wires it, with routes that raise each failure shape. */
+function appWithErrors() {
+  const app = new Hono();
+  app.onError(handleAppError);
+  app.use("*", nonErrorBoundary());
+  app.get("/boom", () => {
+    throw new Error("upstream token abc123 rejected at https://internal.example/db");
+  });
+  app.get("/http-exception/:status", (c) => {
+    throw new HTTPException(Number(c.req.param("status")) as 401, { message: "nope" });
+  });
+  app.get("/non-error", () => {
+    // A thrown non-Error still has to produce a response rather than escaping the boundary.
+    throw "a bare string";
+  });
+  app.get("/ok", (c) => c.json({ ok: true }));
+  return app;
+}
+
+describe("global onError handler", () => {
+  it("REGRESSION: an unexpected throw returns JSON, not Hono's default text/plain body", async () => {
+    const warn = vi.spyOn(console, "error").mockImplementation(() => undefined);
+    const res = await appWithErrors().request("/boom");
+    expect(res.status).toBe(500);
+    // Every other route in this app answers `{ error: ... }`; a text/plain 500 breaks a client's JSON parse
+    // on precisely the response it most needs to read.
+    expect(res.headers.get("content-type")).toContain("application/json");
+    expect(await res.json()).toEqual(INTERNAL_ERROR_BODY);
+    warn.mockRestore();
+  });
+
+  it("REGRESSION: the response body leaks NOTHING — no message, stack, URL or token from the error", async () => {
+    const warn = vi.spyOn(console, "error").mockImplementation(() => undefined);
+    const body = await (await appWithErrors().request("/boom")).text();
+    for (const secret of ["abc123", "internal.example", "upstream token", "rejected", "at /", ".ts:"]) {
+      expect(body).not.toContain(secret);
+    }
+    // The whole body is the fixed code and nothing else.
+    expect(JSON.parse(body)).toEqual({ error: "internal_error" });
+    warn.mockRestore();
+  });
+
+  it("logs ONE structured line the forwarder can classify, with method and path for correlation", async () => {
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    await appWithErrors().request("/boom", { method: "GET" });
+    expect(errors).toHaveLength(1);
+    const logged = JSON.parse(errors[0] as string) as Record<string, unknown>;
+    // Structured, not a raw Error object — the forwarder keys on `level`/`event`.
+    expect(logged).toMatchObject({ level: "error", event: "unhandled_route_error", method: "GET", path: "/boom" });
+    // The detail the BODY withholds lives here, where only an operator sees it.
+    expect(String(logged["message"])).toContain("abc123");
+    expect(typeof logged["stack"]).toBe("string");
+    warn.mockRestore();
+  });
+
+  it("REGRESSION: a query string never reaches the log — `c.req.path` is the matched path only", async () => {
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    await appWithErrors().request("/boom?token=supersecret&id=42");
+    expect(errors[0]).not.toContain("supersecret");
+    expect(JSON.parse(errors[0] as string).path).toBe("/boom");
+    warn.mockRestore();
+  });
+
+  it("REGRESSION: an HTTPException keeps its OWN status — an intentional 4xx never collapses into a 500", async () => {
+    // The single most damaging thing a global handler can get wrong: turning every deliberate 401/403/404
+    // raised by middleware into an opaque server error.
+    const app = appWithErrors();
+    for (const status of [400, 401, 403, 404, 413, 422, 429]) {
+      const res = await app.request(`/http-exception/${status}`);
+      expect({ status, got: res.status }).toEqual({ status, got: status });
+    }
+  });
+
+  it("an HTTPException is NOT logged as an unhandled error — it is an outcome, not a fault", async () => {
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    await appWithErrors().request("/http-exception/401");
+    expect(errors).toEqual([]);
+    warn.mockRestore();
+  });
+
+  it("REGRESSION: a thrown NON-Error is contained — Hono re-throws those, so onError alone never sees them", async () => {
+    // Verified empirically: without nonErrorBoundary the request REJECTS rather than responding, because
+    // compose() only routes `instanceof Error` to onError and re-throws everything else.
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    const res = await appWithErrors().request("/non-error");
+    expect(res.status).toBe(500);
+    expect(await res.json()).toEqual(INTERNAL_ERROR_BODY);
+    const logged = JSON.parse(errors[0] as string) as Record<string, unknown>;
+    expect(logged).toMatchObject({ level: "error", event: "unhandled_non_error_throw", method: "GET", path: "/non-error" });
+    expect(String(logged["thrown"])).toContain("a bare string");
+    warn.mockRestore();
+  });
+
+  it("the non-Error boundary does NOT duplicate onError: an Error thrown below is handled once, by onError", async () => {
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    await appWithErrors().request("/boom");
+    // Exactly one log line, and it is onError's -- the boundary re-throws Errors rather than handling them
+    // a second time, so a change in Hono's own routing would surface instead of being masked.
+    expect(errors).toHaveLength(1);
+    expect(JSON.parse(errors[0] as string).event).toBe("unhandled_route_error");
+    warn.mockRestore();
+  });
+
+  it("a huge or circular thrown value still produces a bounded, loggable line", async () => {
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    const app = new Hono();
+    app.onError(handleAppError);
+    app.use("*", nonErrorBoundary());
+    const circular: Record<string, unknown> = { big: "x".repeat(5000) };
+    circular["self"] = circular;
+    app.get("/circular", () => {
+      throw circular;
+    });
+    const res = await app.request("/circular");
+    expect(res.status).toBe(500);
+    expect(String(JSON.parse(errors[0] as string).thrown).length).toBeLessThanOrEqual(500);
+    warn.mockRestore();
+  });
+
+  it("INVARIANT: a successful route is untouched — the boundary costs nothing on the happy path", async () => {
+    const res = await appWithErrors().request("/ok");
+    expect(res.status).toBe(200);
+    expect(await res.json()).toEqual({ ok: true });
+  });
+
+  it("the boundary's own contract: it re-throws an Error rather than handling it a second time", async () => {
+    // Exercised directly rather than through dispatch, because Hono converts an Error to a response several
+    // levels below and one can never reach the boundary in normal operation. The guard exists so that if
+    // Hono's routing ever changes, the Error surfaces to onError instead of being silently relabelled as an
+    // `unhandled_non_error_throw` -- so the contract is worth pinning even though today nothing triggers it.
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    const context = { req: { method: "GET", path: "/direct" }, json: () => new Response("unused") } as unknown as Parameters<ReturnType<typeof nonErrorBoundary>>[0];
+    const boundary = nonErrorBoundary();
+    const raised = new Error("an Error reached the boundary");
+    await expect(
+      boundary(context, () => {
+        throw raised;
+      }),
+    ).rejects.toBe(raised);
+    // Re-thrown untouched, and NOT logged as a non-Error throw.
+    expect(errors).toEqual([]);
+    warn.mockRestore();
+  });
+
+  it("carriesOwnResponse duck-types on getResponse, so a second hono copy cannot collapse a 4xx into a 500", async () => {
+    // `instanceof HTTPException` would fail across two copies of hono in one module graph -- a real hazard
+    // in a workspace with several packages depending on it. A hand-rolled object carrying getResponse is
+    // treated exactly like a real HTTPException.
+    const app = new Hono();
+    app.onError(handleAppError);
+    app.use("*", nonErrorBoundary());
+    app.get("/foreign", () => {
+      const foreign = new Error("from another hono copy") as Error & { getResponse: () => Response };
+      foreign.getResponse = () => new Response("teapot", { status: 418 });
+      throw foreign;
+    });
+    const errors: string[] = [];
+    const warn = vi.spyOn(console, "error").mockImplementation((line: unknown) => {
+      errors.push(String(line));
+    });
+    const res = await app.request("/foreign");
+    expect(res.status).toBe(418);
+    expect(errors).toEqual([]); // an intentional status is an outcome, not a fault
+    warn.mockRestore();
+  });
+
+  it("REGRESSION: `c.error` is still set, so the PostHog capture middleware keeps seeing failures", async () => {
+    // Hono's compose() assigns `context.error` BEFORE dispatching to whichever handler is installed, so a
+    // custom onError must not break error capture. Asserted through a middleware reading c.error exactly
+    // the way worker-posthog.ts does.
+    const seen: string[] = [];
+    const app = new Hono();
+    app.onError(handleAppError);
+    app.use("*", async (c, next) => {
+      await next();
+      if (c.error) seen.push(c.error.message);
+    });
+    app.get("/boom", () => {
+      throw new Error("captured");
+    });
+    const warn = vi.spyOn(console, "error").mockImplementation(() => undefined);
+    const res = await app.request("/boom");
+    expect(res.status).toBe(500);
+    expect(seen).toEqual(["captured"]);
+    warn.mockRestore();
+  });
+});