diff --git a/packages/opencode/src/altimate/free/client.ts b/packages/opencode/src/altimate/free/client.ts
index 2c7dbff8a..6ac672d7d 100644
--- a/packages/opencode/src/altimate/free/client.ts
+++ b/packages/opencode/src/altimate/free/client.ts
@@ -536,27 +536,49 @@ export function describeRateLimit(
   return undefined
 }
 
-export function describeRequestTooLarge(body?: string): string | undefined {
+// altimate_change start — the generic (no byte-count) request-too-large message is shared by two
+// branches below: the parsed-JSON shape whose message didn't match the byte-count pattern, and
+// the unparseable/empty-body 413 fallback (nginx's raw HTML edge rejection).
+const REQUEST_TOO_LARGE_MESSAGE =
+  "This request is too large for Altimate Base. Start a new session, or switch to another model for this task."
+// altimate_change end
+
+export function describeRequestTooLarge(input: { status?: number; body?: string }): string | undefined {
+  const { status, body } = input
   type Inner = { code?: unknown; message?: unknown; provider_specific_fields?: { error?: Inner } }
   let parsed: { error?: Inner } | undefined
+  let validJson = true
   try {
     parsed = body ? JSON.parse(body) : undefined
   } catch {
-    return undefined
+    validJson = false
+  }
+  if (validJson) {
+    const inner = parsed?.error?.provider_specific_fields?.error
+    const isRequestTooLarge = parsed?.error?.code === "request_too_large" || inner?.code === "request_too_large"
+    if (isRequestTooLarge) {
+      const detail =
+        typeof parsed?.error?.message === "string"
+          ? parsed.error.message
+          : typeof inner?.message === "string"
+            ? inner.message
+            : ""
+      const sizes = detail.match(/Request is (\d+) bytes; the free tier limit is (\d+) bytes/)
+      if (!sizes) return REQUEST_TOO_LARGE_MESSAGE
+      const numbers = ` (${Math.round(Number(sizes[1]) / 1024)}KB against a ${Math.round(Number(sizes[2]) / 1024)}KB limit)`
+      return `This request is too large for Altimate Base${numbers}. Start a new session, or switch to another model for this task.`
+    }
+    // Valid JSON but a shape unrelated to the free-tier byte cap (e.g. another provider's 413,
+    // or a different gateway error entirely) — never rewrite it, regardless of status.
+    if (body) return undefined
   }
-  const inner = parsed?.error?.provider_specific_fields?.error
-  if (parsed?.error?.code !== "request_too_large" && inner?.code !== "request_too_large") return undefined
-  const detail =
-    typeof parsed?.error?.message === "string"
-      ? parsed.error.message
-      : typeof inner?.message === "string"
-        ? inner.message
-        : ""
-  const sizes = detail.match(/Request is (\d+) bytes; the free tier limit is (\d+) bytes/)
-  const numbers = sizes
-    ? ` (${Math.round(Number(sizes[1]) / 1024)}KB against a ${Math.round(Number(sizes[2]) / 1024)}KB limit)`
-    : ""
-  return `This request is too large for Altimate Base${numbers}. Start a new session, or switch to another model for this task.`
+  // altimate_change start — in production, an oversized request is rejected by nginx at the edge
+  // with a raw HTML error page, not LiteLLM's JSON `request_too_large` body. That body will never
+  // parse, so the friendly message must key off the actual HTTP status instead of a JSON shape
+  // that this failure mode can never produce.
+  if (status === 413) return REQUEST_TOO_LARGE_MESSAGE
+  // altimate_change end
+  return undefined
 }
 
 export * as FreeTier from "./client"
diff --git a/packages/opencode/src/provider/error.ts b/packages/opencode/src/provider/error.ts
index 1de0f4ceb..aeea86f1f 100644
--- a/packages/opencode/src/provider/error.ts
+++ b/packages/opencode/src/provider/error.ts
@@ -333,7 +333,10 @@ export namespace ProviderError {
     // The gateway's fixed request-byte cap is not a context overflow. Retrying compaction can
     // never help when system instructions and tool schemas alone exceed it.
     if (String(input.providerID) === FreeTier.PROVIDER_ID && input.error.statusCode === 413) {
-      const described = FreeTier.describeRequestTooLarge(input.error.responseBody)
+      const described = FreeTier.describeRequestTooLarge({
+        status: input.error.statusCode,
+        body: input.error.responseBody,
+      })
       if (described) {
         return {
           type: "api_error",
diff --git a/packages/opencode/src/provider/provider.ts b/packages/opencode/src/provider/provider.ts
index ea35e36f3..e74e5500c 100644
--- a/packages/opencode/src/provider/provider.ts
+++ b/packages/opencode/src/provider/provider.ts
@@ -393,6 +393,9 @@ export namespace Provider {
           // authorizedFetch. Provider options are serialized by public provider APIs.
           apiKey: FreeTier.MANAGED_API_KEY_PLACEHOLDER,
           fetch: FreeTier.authorizedFetch,
+          // BUG FIX: without this, a hung gateway response never times out client-side, unlike
+          // the openai loader below which already sets this.
+          headerTimeout: OPENAI_HEADER_TIMEOUT_DEFAULT,
         },
       }
     },
diff --git a/packages/opencode/test/altimate/altimate-base-error-surfacing.test.ts b/packages/opencode/test/altimate/altimate-base-error-surfacing.test.ts
index e35b041a0..8d0f97c37 100644
--- a/packages/opencode/test/altimate/altimate-base-error-surfacing.test.ts
+++ b/packages/opencode/test/altimate/altimate-base-error-surfacing.test.ts
@@ -150,7 +150,7 @@ describe("malformed JSON response body during inference", () => {
     // The Altimate-Base-specific error mappers must also degrade gracefully on this same malformed
     // body — both already catch a `JSON.parse` failure and return `undefined` rather than crashing.
     expect(FreeTier.describeRateLimit({ body: raw })).toBeUndefined()
-    expect(FreeTier.describeRequestTooLarge(raw)).toBeUndefined()
+    expect(FreeTier.describeRequestTooLarge({ status: response.status, body: raw })).toBeUndefined()
   })
 })
 
diff --git a/packages/opencode/test/altimate/altimate-base-rate-limit-messages.test.ts b/packages/opencode/test/altimate/altimate-base-rate-limit-messages.test.ts
index e7d5de5af..18702eb07 100644
--- a/packages/opencode/test/altimate/altimate-base-rate-limit-messages.test.ts
+++ b/packages/opencode/test/altimate/altimate-base-rate-limit-messages.test.ts
@@ -196,7 +196,7 @@ describe("describeRequestTooLarge — via the fake gateway (413 request_too_larg
     const response = await chat()
     expect(response.status).toBe(413)
 
-    const described = FreeTier.describeRequestTooLarge(await response.text())
+    const described = FreeTier.describeRequestTooLarge({ status: response.status, body: await response.text() })
     expect(described).toBe(
       "This request is too large for Altimate Base (175KB against a 125KB limit). Start a new session, or switch to another model for this task.",
     )
@@ -208,7 +208,7 @@ describe("describeRequestTooLarge — via the fake gateway (413 request_too_larg
     const response = await chat()
     expect(response.status).toBe(413)
 
-    const described = FreeTier.describeRequestTooLarge(await response.text())
+    const described = FreeTier.describeRequestTooLarge({ status: response.status, body: await response.text() })
     expect(described).toBe(
       "This request is too large for Altimate Base (977KB against a 488KB limit). Start a new session, or switch to another model for this task.",
     )
@@ -227,7 +227,7 @@ describe("describeRequestTooLarge — via the fake gateway (413 request_too_larg
     expect(body.error.provider_specific_fields.error.code).toBe("request_too_large")
 
     // 50000/1024 = 48.828125 -> round 49. 40000/1024 = 39.0625 -> round 39.
-    const described = FreeTier.describeRequestTooLarge(JSON.stringify(body))
+    const described = FreeTier.describeRequestTooLarge({ status: response.status, body: JSON.stringify(body) })
     expect(described).toBe(
       "This request is too large for Altimate Base (49KB against a 39KB limit). Start a new session, or switch to another model for this task.",
     )
@@ -236,19 +236,20 @@ describe("describeRequestTooLarge — via the fake gateway (413 request_too_larg
 
 describe("describeRequestTooLarge — pure-function edge cases FakeGateway's ChatMode cannot express", () => {
   test("absent body returns undefined", () => {
-    expect(FreeTier.describeRequestTooLarge(undefined)).toBeUndefined()
+    expect(FreeTier.describeRequestTooLarge({})).toBeUndefined()
   })
 
-  test("unparseable JSON body returns undefined", () => {
-    expect(FreeTier.describeRequestTooLarge("{not json")).toBeUndefined()
+  test("unparseable JSON body returns undefined when the status isn't 413", () => {
+    expect(FreeTier.describeRequestTooLarge({ body: "{not json" })).toBeUndefined()
   })
 
   test("a 413 without the request_too_large code (an unrelated provider 413) returns undefined", () => {
     // Falls through to error.ts's generic context_overflow handling instead of the
     // Altimate-Base-specific rewrite — request_too_large and context overflow are distinct
-    // gateway error codes.
+    // gateway error codes. Valid, parseable JSON with a different shape must never be rewritten,
+    // even though the status is 413 — that's what separates this from the HTML-body fallback below.
     const body = JSON.stringify({ error: { code: "some_other_413", message: "payload too large" } })
-    expect(FreeTier.describeRequestTooLarge(body)).toBeUndefined()
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBeUndefined()
   })
 
   test("the outer error.code (not the nested provider_specific_fields shape) also matches", () => {
@@ -256,7 +257,7 @@ describe("describeRequestTooLarge — pure-function edge cases FakeGateway's Cha
       error: { code: "request_too_large", message: "Request is 300000 bytes; the free tier limit is 100000 bytes." },
     })
     // 300000/1024 = 292.96875 -> round 293. 100000/1024 = 97.65625 -> round 98.
-    expect(FreeTier.describeRequestTooLarge(body)).toBe(
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBe(
       "This request is too large for Altimate Base (293KB against a 98KB limit). Start a new session, or switch to another model for this task.",
     )
   })
@@ -265,14 +266,14 @@ describe("describeRequestTooLarge — pure-function edge cases FakeGateway's Cha
     const body = JSON.stringify({
       error: { code: "request_too_large", message: "Payload rejected: too large for this tier." },
     })
-    expect(FreeTier.describeRequestTooLarge(body)).toBe(
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBe(
       "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
     )
   })
 
   test("request_too_large with no message at all on either the outer or inner error omits the KB parenthetical", () => {
     const body = JSON.stringify({ error: { code: "request_too_large" } })
-    expect(FreeTier.describeRequestTooLarge(body)).toBe(
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBe(
       "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
     )
   })
@@ -292,8 +293,44 @@ describe("describeRequestTooLarge — pure-function edge cases FakeGateway's Cha
       },
     })
     // 250000/1024 = 244.140625 -> round 244. 128000/1024 = 125 exactly.
-    expect(FreeTier.describeRequestTooLarge(body)).toBe(
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBe(
       "This request is too large for Altimate Base (244KB against a 125KB limit). Start a new session, or switch to another model for this task.",
     )
   })
 })
+
+// BUG FIX: in production, nginx rejects oversized requests at the edge with a raw HTML error
+// page — not LiteLLM's JSON `request_too_large` shape. The unparseable/empty body previously fell
+// through to `undefined` (a generic fallback), even though the status line already says 413.
+describe("describeRequestTooLarge — 413 with a body that never parses (nginx edge rejection)", () => {
+  test("a 413 with a raw HTML body returns the friendly fallback message, not undefined", () => {
+    const html =
+      "<html>\n<head><title>413 Request Entity Too Large</title></head>\n<body>\n<center>413 Request Entity Too Large</center>\n<hr><center>nginx</center>\n</body>\n</html>"
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body: html })).toBe(
+      "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
+    )
+  })
+
+  test("a 413 with an empty body returns the same friendly fallback message", () => {
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body: "" })).toBe(
+      "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
+    )
+    expect(FreeTier.describeRequestTooLarge({ status: 413 })).toBe(
+      "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
+    )
+  })
+
+  test("a 413 with the known JSON shape still returns the specific byte-count message, not the generic fallback", () => {
+    const body = JSON.stringify({
+      error: { code: "request_too_large", message: "Request is 179608 bytes; the free tier limit is 128000 bytes." },
+    })
+    expect(FreeTier.describeRequestTooLarge({ status: 413, body })).toBe(
+      "This request is too large for Altimate Base (175KB against a 125KB limit). Start a new session, or switch to another model for this task.",
+    )
+  })
+
+  test("an HTML body on a non-413 status still returns undefined (status is what gates the fallback)", () => {
+    const html = "<html><body>502 Bad Gateway</body></html>"
+    expect(FreeTier.describeRequestTooLarge({ status: 502, body: html })).toBeUndefined()
+  })
+})
diff --git a/packages/opencode/test/provider/error.test.ts b/packages/opencode/test/provider/error.test.ts
index df94588d0..9a3ca0a14 100644
--- a/packages/opencode/test/provider/error.test.ts
+++ b/packages/opencode/test/provider/error.test.ts
@@ -466,4 +466,24 @@ describe("ProviderError.parseAPICallError: Altimate Base isolation", () => {
     })
     expect(result.type).toBe("context_overflow")
   })
+
+  // altimate_change start — BUG FIX: production rejects oversized requests with nginx's raw HTML
+  // edge page, not LiteLLM's JSON body. That body never parses, so this must still resolve to the
+  // friendly Altimate Base message instead of falling through to the generic context_overflow path.
+  test("a 413 with an unparseable HTML body (nginx edge rejection) still gets the friendly Altimate Base message", () => {
+    const htmlBody =
+      "<html>\n<head><title>413 Request Entity Too Large</title></head>\n<body>\n<center>413 Request Entity Too Large</center>\n<hr><center>nginx</center>\n</body>\n</html>"
+    const result = ProviderError.parseAPICallError({
+      providerID: "altimate-free" as any,
+      error: makeAPICallError({ message: "Payload Too Large", statusCode: 413, responseBody: htmlBody }),
+    })
+    expect(result.type).toBe("api_error")
+    if (result.type === "api_error") {
+      expect(result.message).toBe(
+        "This request is too large for Altimate Base. Start a new session, or switch to another model for this task.",
+      )
+      expect(result.isRetryable).toBe(false)
+    }
+  })
+  // altimate_change end
 })
diff --git a/packages/opencode/test/provider/provider.test.ts b/packages/opencode/test/provider/provider.test.ts
index 8b5fc9420..1cb1537a7 100644
--- a/packages/opencode/test/provider/provider.test.ts
+++ b/packages/opencode/test/provider/provider.test.ts
@@ -80,6 +80,9 @@ test("Altimate Base is pinned to the hosted model contract without affecting oth
         expect(base.env).toEqual([])
         expect(base.options.baseURL).toBe(`${ALTIMATE_BASE_GATEWAY_URL}/v1`)
         expect(base.options.apiKey).toBe(FreeTier.MANAGED_API_KEY_PLACEHOLDER)
+        // BUG FIX: without a client-side header timeout, a hung gateway response never resolves
+        // or rejects — the CLI just hangs. This mirrors the openai loader's default.
+        expect(base.options.headerTimeout).toBe(10_000)
         expect(JSON.stringify(base)).not.toContain("sk-altimate-base")
 
         const model = base.models[FreeTier.MODEL_ID]