diff --git a/changelog.d/features/6354-per-model-timeout.md b/changelog.d/features/6354-per-model-timeout.md
new file mode 100644
index 0000000000..9537a5d0f1
--- /dev/null
+++ b/changelog.d/features/6354-per-model-timeout.md
@@ -0,0 +1 @@
+- feat(sse): configurable per-model upstream header-response timeout override, precedence model > provider > global; applied to codex reasoning-heavy tiers (gpt-5.5-high/xhigh, gpt-5.6-\*-high/xhigh) (#6354)
diff --git a/open-sse/config/providerModels.ts b/open-sse/config/providerModels.ts
index ac8c0aa48d..6dc6ff9654 100644
--- a/open-sse/config/providerModels.ts
+++ b/open-sse/config/providerModels.ts
@@ -73,6 +73,21 @@ export function getModelsByProviderId(providerId: string): RegistryModel[] {
   return PROVIDER_MODELS[alias] || [];
 }
 
+/**
+ * Model-level upstream header-response timeout override, when the registry
+ * entry for `modelId` sets one (#6354). Returns `undefined` when the model
+ * isn't found or has no override, so callers can fall through to the
+ * provider-level/global defaults unchanged.
+ */
+export function getModelTimeoutMs(aliasOrId: string, modelId: string): number | undefined {
+  // Callers (e.g. chatCore's timeout resolution) pass the raw provider id
+  // ("codex"), not the public alias ("cx") that PROVIDER_MODELS is keyed by
+  // — resolve id→alias the same way getProviderModels()/getModelsByProviderId()
+  // do, so the override actually resolves (#6354).
+  const alias = PROVIDER_ID_TO_ALIAS[aliasOrId] || aliasOrId;
+  return getProviderModel(alias, modelId)?.timeoutMs;
+}
+
 const CLAUDE_MODEL_PATTERN = /(?:^|[\/._-])claude(?:[._-]|$)/;
 const CLAUDE_MAX_EFFORT_UNSUPPORTED_FAMILY_PATTERNS = [/(?:^|[\/._-])haiku(?:[._-]|$)/] as const;
 const ANTHROPIC_COMPATIBLE_PREFIX = "anthropic-compatible-";
diff --git a/open-sse/config/providers/registry/codex/index.ts b/open-sse/config/providers/registry/codex/index.ts
index 01a43c71fb..7d6fee4557 100644
--- a/open-sse/config/providers/registry/codex/index.ts
+++ b/open-sse/config/providers/registry/codex/index.ts
@@ -43,11 +43,15 @@ export const codexProvider: RegistryEntry = {
       id: "gpt-5.6-sol-xhigh",
       name: "GPT 5.6 Sol (xHigh)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-sol-high",
       name: "GPT 5.6 Sol (High)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-sol-medium",
@@ -78,11 +82,15 @@ export const codexProvider: RegistryEntry = {
       id: "gpt-5.6-terra-xhigh",
       name: "GPT 5.6 Terra (xHigh)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-terra-high",
       name: "GPT 5.6 Terra (High)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-terra-medium",
@@ -108,11 +116,15 @@ export const codexProvider: RegistryEntry = {
       id: "gpt-5.6-luna-xhigh",
       name: "GPT 5.6 Luna (xHigh)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-luna-high",
       name: "GPT 5.6 Luna (High)",
       ...GPT_5_6_CODEX_CAPABILITIES,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.6-luna-medium",
@@ -149,6 +161,8 @@ export const codexProvider: RegistryEntry = {
       // #6191: input cap per reporter; TODO confirm exact value
       maxInputTokens: 272000,
       maxOutputTokens: 128000,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.5-high",
@@ -158,6 +172,8 @@ export const codexProvider: RegistryEntry = {
       // #6191: input cap per reporter; TODO confirm exact value
       maxInputTokens: 272000,
       maxOutputTokens: 128000,
+      // #6354: reasoning-heavy tier — more header-wait room than the global default.
+      timeoutMs: 1200000,
     },
     {
       id: "gpt-5.5-medium",
diff --git a/open-sse/config/providers/shared.ts b/open-sse/config/providers/shared.ts
index d022b86169..fd9d809381 100644
--- a/open-sse/config/providers/shared.ts
+++ b/open-sse/config/providers/shared.ts
@@ -71,6 +71,9 @@ export interface RegistryModel {
    * reasoning_content instead of failing with a DeepSeek 400 (#2900).
    */
   interleavedField?: string;
+  /** Per-model upstream header-response timeout override — precedes
+   *  `RegistryEntry.timeoutMs` and the global `FETCH_TIMEOUT_MS` (#6354). */
+  timeoutMs?: number;
 }
 
 // Reasoning models reject temperature, top_p, penalties, logprobs, n.
diff --git a/open-sse/handlers/chatCore/upstreamTimeouts.ts b/open-sse/handlers/chatCore/upstreamTimeouts.ts
index 6f3636bb1f..0c63c53346 100644
--- a/open-sse/handlers/chatCore/upstreamTimeouts.ts
+++ b/open-sse/handlers/chatCore/upstreamTimeouts.ts
@@ -1,4 +1,5 @@
 import { FETCH_TIMEOUT_MS } from "../../config/constants.ts";
+import { getModelTimeoutMs } from "../../config/providerModels.ts";
 import {
   getLoggedInputTokens,
   getLoggedOutputTokens,
@@ -62,7 +63,16 @@ export function computeBillableTokens(usage: unknown): number {
   return getLoggedInputTokens(usage) + getLoggedOutputTokens(usage) + getReasoningTokens(usage);
 }
 
-export function getExecutorTimeoutMs(executor: unknown): number {
+/** Resolves the model-level `timeoutMs` registry override, when both
+ *  `provider` and `model` are known and the model registers one (#6354). */
+function resolveModelTimeoutOverride(provider?: string, model?: string): number | undefined {
+  if (!provider || !model) return undefined;
+  const override = getModelTimeoutMs(provider, model);
+  if (typeof override !== "number" || !Number.isFinite(override)) return undefined;
+  return Math.max(0, Math.floor(override));
+}
+
+function resolveProviderTimeoutMs(executor: unknown): number {
   const getTimeoutMs = (executor as { getTimeoutMs?: () => unknown } | null)?.getTimeoutMs;
   if (typeof getTimeoutMs !== "function") return FETCH_TIMEOUT_MS;
 
@@ -75,6 +85,19 @@ export function getExecutorTimeoutMs(executor: unknown): number {
   }
 }
 
+/**
+ * Resolves the upstream header-response timeout in precedence order:
+ * model-level override (registry `RegistryModel.timeoutMs`) → provider-level
+ * override (`executor.getTimeoutMs()`) → global `FETCH_TIMEOUT_MS` default.
+ * `provider`/`model` are optional so existing single-argument call sites
+ * keep resolving to the provider/global chain unchanged (#6354).
+ */
+export function getExecutorTimeoutMs(executor: unknown, provider?: string, model?: string): number {
+  const modelOverride = resolveModelTimeoutOverride(provider, model);
+  if (modelOverride !== undefined) return modelOverride;
+  return resolveProviderTimeoutMs(executor);
+}
+
 export function normalizeExecutorResult(
   result:
     | Response
@@ -111,7 +134,7 @@ export async function executeWithUpstreamStartTimeout<T>({
   log?: { warn?: (tag: string, message: string) => void } | null;
   execute: (signal: AbortSignal) => Promise<T>;
 }): Promise<T> {
-  const timeoutMs = getExecutorTimeoutMs(executor);
+  const timeoutMs = getExecutorTimeoutMs(executor, provider, model);
   if (timeoutMs <= 0) return execute(signal);
   if (signal.aborted) throw createAbortError(signal);
 
diff --git a/tests/unit/upstream-timeout-model-override.test.ts b/tests/unit/upstream-timeout-model-override.test.ts
new file mode 100644
index 0000000000..ec3a05b43f
--- /dev/null
+++ b/tests/unit/upstream-timeout-model-override.test.ts
@@ -0,0 +1,49 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { getExecutorTimeoutMs } from "../../open-sse/handlers/chatCore/upstreamTimeouts.ts";
+import { FETCH_TIMEOUT_MS } from "../../open-sse/config/constants.ts";
+import { getModelTimeoutMs } from "../../open-sse/config/providerModels.ts";
+
+test("model-level timeoutMs wins over provider-level and global", () => {
+  const executor = { getTimeoutMs: () => 30000 };
+  const result = getExecutorTimeoutMs(executor, "codex", "gpt-5.5-high");
+  assert.equal(result, 1200000);
+  assert.notEqual(result, 30000);
+  assert.notEqual(result, FETCH_TIMEOUT_MS);
+});
+
+test("provider-level timeoutMs still wins over global when no model override exists (regression guard)", () => {
+  const executor = { getTimeoutMs: () => 45000 };
+  // No model registered for this provider/model combo -> no model override,
+  // must still fall back to the provider-level executor.getTimeoutMs().
+  const result = getExecutorTimeoutMs(executor, "codex", "gpt-5.5-medium");
+  assert.equal(result, 45000);
+});
+
+test("global FETCH_TIMEOUT_MS remains the fallback with neither override", () => {
+  const executor = {};
+  const result = getExecutorTimeoutMs(executor, "codex", "gpt-5.5-medium");
+  assert.equal(result, FETCH_TIMEOUT_MS);
+});
+
+test("getExecutorTimeoutMs is backward compatible when called with only an executor", () => {
+  const executor = { getTimeoutMs: () => 5000 };
+  assert.equal(getExecutorTimeoutMs(executor), 5000);
+});
+
+test("gpt-5.5-high resolves to its override; gpt-5.5-medium resolves to the provider/global default (reported-scenario guard)", () => {
+  const executor = {};
+  const highResult = getExecutorTimeoutMs(executor, "codex", "gpt-5.5-high");
+  const mediumResult = getExecutorTimeoutMs(executor, "codex", "gpt-5.5-medium");
+  assert.equal(highResult, 1200000);
+  assert.equal(mediumResult, FETCH_TIMEOUT_MS);
+  assert.notEqual(highResult, mediumResult);
+});
+
+test("getModelTimeoutMs returns the registered override for reasoning-heavy codex tiers", () => {
+  assert.equal(getModelTimeoutMs("codex", "gpt-5.5-high"), 1200000);
+  assert.equal(getModelTimeoutMs("codex", "gpt-5.5-xhigh"), 1200000);
+  assert.equal(getModelTimeoutMs("codex", "gpt-5.5-medium"), undefined);
+  assert.equal(getModelTimeoutMs("codex", "nonexistent-model"), undefined);
+});