diff --git a/actions/setup/js/awf_reflect.cjs b/actions/setup/js/awf_reflect.cjs
index 3fd24f6f236..2e0e1d754f4 100644
--- a/actions/setup/js/awf_reflect.cjs
+++ b/actions/setup/js/awf_reflect.cjs
@@ -409,6 +409,56 @@ function inferProviderTypeForModel(endpointProvider, modelName, modelsJson) {
   return "openai";
 }
 
+/**
+ * Infer the wire API for a given model ID from the models catalog.
+ * Returns "responses" when the model's catalog entry specifies `wire_api: "responses"`,
+ * or "completions" as the safe default for all other cases.
+ *
+ * @param {string} modelId
+ * @param {object | null | undefined} modelsJson
+ * @returns {"completions" | "responses"}
+ */
+function inferWireApiForModel(modelId, modelsJson) {
+  const model = String(modelId || "")
+    .toLowerCase()
+    .trim();
+  if (!model || modelsJson == null) return "completions";
+
+  const providers = modelsJson && typeof modelsJson === "object" && !Array.isArray(modelsJson) ? modelsJson.providers : null;
+  if (providers && typeof providers === "object") {
+    for (const providerData of Object.values(providers)) {
+      const models = providerData && typeof providerData === "object" ? providerData.models : null;
+      if (models && typeof models === "object") {
+        for (const [catalogModel, catalogEntry] of Object.entries(models)) {
+          if (
+            String(catalogModel || "")
+              .toLowerCase()
+              .trim() === model
+          ) {
+            if (catalogEntry && catalogEntry.wire_api === "responses") return "responses";
+          }
+        }
+      }
+    }
+  }
+
+  return "completions";
+}
+
+/**
+ * Infer the wire API required by a set of model IDs.
+ * Returns "responses" if any model in the list requires it, "completions" otherwise.
+ * This is used when a session may involve multiple models (e.g. main model + sub-agent models).
+ *
+ * @param {string[]} modelIds
+ * @param {object | null | undefined} modelsJson
+ * @returns {"completions" | "responses"}
+ */
+function inferWireApiForModels(modelIds, modelsJson) {
+  if (!Array.isArray(modelIds)) return "completions";
+  return modelIds.some(id => inferWireApiForModel(id, modelsJson) === "responses") ? "responses" : "completions";
+}
+
 /**
  * Resolve Copilot SDK BYOK custom provider configuration from AWF /reflect data.
  * Chooses a configured endpoint and maps it to a provider base URL and type.
@@ -424,7 +474,7 @@ function inferProviderTypeForModel(endpointProvider, modelName, modelsJson) {
  *   modelsJson?: object | null,
  *   logger?: (msg: string) => void,
  * }} [options]
- * @returns {{ model: string, provider: { type: "openai" | "azure" | "anthropic", baseUrl: string } } | null}
+ * @returns {{ model: string, provider: { type: "openai" | "azure" | "anthropic", baseUrl: string, wireApi?: "completions" | "responses" } } | null}
  */
 function resolveCopilotSDKCustomProviderFromReflect(options) {
   const configuredModel = typeof options?.model === "string" ? options.model.trim() : "";
@@ -476,10 +526,11 @@ function resolveCopilotSDKCustomProviderFromReflect(options) {
   }
 
   const providerType = inferProviderTypeForModel(String(endpoint.provider || ""), model, options?.modelsJson ?? null);
+  const wireApi = inferWireApiForModel(model, options?.modelsJson ?? null);
   logger(`sdk-mode: custom provider resolved from awf-reflect (provider=${String(endpoint.provider || "unknown")} type=${providerType} baseUrl=${baseUrl} model=${model})`);
   return {
     model,
-    provider: { type: providerType, baseUrl },
+    provider: { type: providerType, baseUrl, ...(wireApi !== "completions" ? { wireApi } : {}) },
   };
 }
 
@@ -498,6 +549,8 @@ if (typeof module !== "undefined" && module.exports) {
     fetchAWFReflect,
     fetchModelsFromUrl,
     inferProviderTypeForModel,
+    inferWireApiForModel,
+    inferWireApiForModels,
     resolveCopilotSDKCustomProviderFromReflect,
   };
 }
diff --git a/actions/setup/js/awf_reflect.test.cjs b/actions/setup/js/awf_reflect.test.cjs
index bd8ad68f1cc..501690f1175 100644
--- a/actions/setup/js/awf_reflect.test.cjs
+++ b/actions/setup/js/awf_reflect.test.cjs
@@ -19,6 +19,8 @@ const {
   fetchAWFReflect,
   fetchModelsFromUrl,
   inferProviderTypeForModel,
+  inferWireApiForModel,
+  inferWireApiForModels,
   resolveCopilotSDKCustomProviderFromReflect,
 } = require("./awf_reflect.cjs");
 
@@ -396,6 +398,90 @@ describe("awf_reflect.cjs", () => {
     });
   });
 
+  describe("inferWireApiForModel", () => {
+    it("returns 'completions' for an unknown model", () => {
+      expect(inferWireApiForModel("unknown-model", null)).toBe("completions");
+      expect(inferWireApiForModel("unknown-model", {})).toBe("completions");
+    });
+
+    it("returns 'completions' for a model without a wire_api field", () => {
+      const modelsJson = {
+        providers: {
+          openai: { models: { "gpt-5.4": { provider_type: "openai", cost: {} } } },
+        },
+      };
+      expect(inferWireApiForModel("gpt-5.4", modelsJson)).toBe("completions");
+    });
+
+    it("returns 'responses' for gpt-5.5 which has wire_api: responses", () => {
+      const modelsJson = {
+        providers: {
+          openai: { models: { "gpt-5.5": { provider_type: "openai", wire_api: "responses", cost: {} } } },
+        },
+      };
+      expect(inferWireApiForModel("gpt-5.5", modelsJson)).toBe("responses");
+    });
+
+    it("finds a model across multiple providers", () => {
+      const modelsJson = {
+        providers: {
+          "github-copilot": { models: { "gpt-5.4": { provider_type: "openai", cost: {} } } },
+          openai: { models: { "gpt-5.5": { provider_type: "openai", wire_api: "responses", cost: {} } } },
+        },
+      };
+      expect(inferWireApiForModel("gpt-5.5", modelsJson)).toBe("responses");
+      expect(inferWireApiForModel("gpt-5.4", modelsJson)).toBe("completions");
+    });
+
+    it("returns 'completions' when modelsJson is null or undefined", () => {
+      expect(inferWireApiForModel("gpt-5.5", null)).toBe("completions");
+      expect(inferWireApiForModel("gpt-5.5", undefined)).toBe("completions");
+    });
+  });
+
+  describe("inferWireApiForModels", () => {
+    const modelsJson = {
+      providers: {
+        openai: {
+          models: {
+            "gpt-5.4": { provider_type: "openai", cost: {} },
+            "gpt-5.5": { provider_type: "openai", wire_api: "responses", cost: {} },
+            "gpt-5.5-pro": { provider_type: "openai", wire_api: "responses", cost: {} },
+          },
+        },
+      },
+    };
+
+    it("returns 'completions' when no model needs responses", () => {
+      expect(inferWireApiForModels(["gpt-5.4"], modelsJson)).toBe("completions");
+      expect(inferWireApiForModels(["gpt-5.4", "claude-sonnet-4.5"], modelsJson)).toBe("completions");
+    });
+
+    it("returns 'responses' when any model needs it", () => {
+      expect(inferWireApiForModels(["gpt-5.5"], modelsJson)).toBe("responses");
+      expect(inferWireApiForModels(["gpt-5.4", "gpt-5.5"], modelsJson)).toBe("responses");
+      expect(inferWireApiForModels(["gpt-5.5-pro", "gpt-5.4"], modelsJson)).toBe("responses");
+    });
+
+    it("returns 'responses' when a subagent model requires it and the main model does not", () => {
+      // Simulates main model = gpt-5.4 (completions), subagent = gpt-5.5 (responses)
+      expect(inferWireApiForModels(["gpt-5.4", "gpt-5.5"], modelsJson)).toBe("responses");
+    });
+
+    it("returns 'completions' for an empty list", () => {
+      expect(inferWireApiForModels([], modelsJson)).toBe("completions");
+    });
+
+    it("returns 'completions' when modelsJson is null", () => {
+      expect(inferWireApiForModels(["gpt-5.5"], null)).toBe("completions");
+    });
+
+    it("returns 'completions' for a non-array input", () => {
+      expect(inferWireApiForModels(null, modelsJson)).toBe("completions");
+      expect(inferWireApiForModels(undefined, modelsJson)).toBe("completions");
+    });
+  });
+
   describe("resolveCopilotSDKCustomProviderFromReflect", () => {
     it("resolves provider baseUrl and model from port when models_url is absent", () => {
       const reflectData = {
@@ -517,5 +603,33 @@ describe("awf_reflect.cjs", () => {
       expect(result).toBeNull();
       expect(logs.some(l => l.includes("no reflect data provided"))).toBe(true);
     });
+
+    it("omits wireApi from provider when model uses default completions API", () => {
+      const reflectData = {
+        endpoints: [{ provider: "copilot", port: 10002, configured: true, models: ["gpt-5.4"] }],
+      };
+      const modelsJson = {
+        providers: {
+          openai: { models: { "gpt-5.4": { provider_type: "openai", cost: {} } } },
+        },
+      };
+      const result = resolveCopilotSDKCustomProviderFromReflect({ reflectData, model: "gpt-5.4", modelsJson });
+      expect(result).not.toBeNull();
+      expect(result.provider).not.toHaveProperty("wireApi");
+    });
+
+    it("includes wireApi: 'responses' in provider when model requires responses API", () => {
+      const reflectData = {
+        endpoints: [{ provider: "copilot", port: 10002, configured: true, models: ["gpt-5.5"] }],
+      };
+      const modelsJson = {
+        providers: {
+          openai: { models: { "gpt-5.5": { provider_type: "openai", wire_api: "responses", cost: {} } } },
+        },
+      };
+      const result = resolveCopilotSDKCustomProviderFromReflect({ reflectData, model: "gpt-5.5", modelsJson });
+      expect(result).not.toBeNull();
+      expect(result.provider.wireApi).toBe("responses");
+    });
   });
 });
diff --git a/actions/setup/js/copilot_harness.cjs b/actions/setup/js/copilot_harness.cjs
index 980a9f654c3..de2fbdf40b8 100644
--- a/actions/setup/js/copilot_harness.cjs
+++ b/actions/setup/js/copilot_harness.cjs
@@ -57,6 +57,7 @@ const {
   fetchAWFReflect,
   fetchModelsFromUrl,
   inferProviderTypeForModel,
+  inferWireApiForModels,
   resolveCopilotSDKCustomProviderFromReflect,
 } = require("./awf_reflect.cjs");
 const { runSafeOutputsCLI, buildMissingToolAlternatives, emitMissingToolPermissionIssue, emitInfrastructureIncomplete, hasExpectedSafeOutputs, hasNoopInSafeOutputs } = require("./safeoutputs_cli.cjs");
@@ -659,6 +660,50 @@ function resolvePromptFileArgs(args) {
   return resolvedArgs;
 }
 
+/**
+ * Extract the `model:` value from a YAML frontmatter block at the start of a string.
+ * Returns an empty string if the content has no frontmatter or no model field.
+ *
+ * @param {string} content
+ * @returns {string}
+ */
+function extractModelFromAgentFrontmatter(content) {
+  const match = String(content || "").match(/^---\r?\n([\s\S]*?)\n---/);
+  if (!match) return "";
+  const modelMatch = match[1].match(/^model:\s*(.+)$/m);
+  return modelMatch ? modelMatch[1].trim() : "";
+}
+
+/**
+ * Scan `.github/agents/*.agent.md` files under `baseDir` and return the model
+ * identifiers declared in each file's YAML frontmatter.
+ * Silently ignores missing directory or unreadable files.
+ *
+ * @param {string} baseDir - Workspace root (typically GITHUB_WORKSPACE).
+ * @returns {string[]}
+ */
+function collectSubAgentModelIds(baseDir) {
+  if (!baseDir) return [];
+  const agentsDir = require("path").join(baseDir, ".github", "agents");
+  const modelIds = [];
+  try {
+    const files = fs.readdirSync(agentsDir);
+    for (const file of files) {
+      if (!file.endsWith(".agent.md")) continue;
+      try {
+        const content = fs.readFileSync(require("path").join(agentsDir, file), "utf8");
+        const model = extractModelFromAgentFrontmatter(content);
+        if (model) modelIds.push(model);
+      } catch {
+        // ignore unreadable files
+      }
+    }
+  } catch {
+    // directory doesn't exist or is unreadable — not an error
+  }
+  return modelIds;
+}
+
 /**
  * Main entry point: run copilot with retry logic for partially-executed sessions.
  */
@@ -716,6 +761,7 @@ async function main() {
   // cannot be resolved so retries are not wasted on a misconfigured environment.
   let providerBaseUrl = "";
   let providerType = "openai";
+  let providerWireApi = "";
   let resolvedModel = "";
   if (copilotSDKMode) {
     const configuredModel = process.env.COPILOT_MODEL || "";
@@ -728,8 +774,25 @@ async function main() {
     }
     providerBaseUrl = customProvider.provider.baseUrl;
     providerType = customProvider.provider.type || "openai";
+    providerWireApi = customProvider.provider.wireApi || "";
     resolvedModel = customProvider.model;
     log(`copilot-sdk driver mode: BYOK provider resolved (baseUrl=${providerBaseUrl} type=${providerType} model=${resolvedModel})`);
+
+    // Upgrade wireApi to "responses" if any inline sub-agent model requires it.
+    // The session-level provider wireApi applies to all model calls, including sub-agents,
+    // so we must use "responses" whenever any model in the session needs it.
+    if (providerWireApi !== "responses") {
+      const workspace = process.env.GITHUB_WORKSPACE || "";
+      const subAgentModelIds = collectSubAgentModelIds(workspace);
+      if (subAgentModelIds.length > 0) {
+        log(`copilot-sdk driver mode: found ${subAgentModelIds.length} sub-agent model(s): ${subAgentModelIds.join(", ")}`);
+        const effectiveWireApi = inferWireApiForModels([resolvedModel, ...subAgentModelIds], modelsJson);
+        if (effectiveWireApi === "responses") {
+          log(`copilot-sdk driver mode: upgrading wireApi to "${effectiveWireApi}" to support sub-agent model(s)`);
+          providerWireApi = effectiveWireApi;
+        }
+      }
+    }
   }
 
   // Merge SDK env additions into the child process env only when the SDK helper
@@ -746,6 +809,7 @@ async function main() {
         COPILOT_CONNECTION_TOKEN: copilotConnectionToken,
         GH_AW_COPILOT_SDK_PROVIDER_BASE_URL: providerBaseUrl,
         GH_AW_COPILOT_SDK_PROVIDER_TYPE: providerType,
+        GH_AW_COPILOT_SDK_PROVIDER_WIRE_API: providerWireApi,
         COPILOT_MODEL: resolvedModel,
       }
     : sdkEnv;
diff --git a/actions/setup/js/copilot_sdk_driver.cjs b/actions/setup/js/copilot_sdk_driver.cjs
index 9d3bd01d458..825b1a47beb 100644
--- a/actions/setup/js/copilot_sdk_driver.cjs
+++ b/actions/setup/js/copilot_sdk_driver.cjs
@@ -13,6 +13,7 @@
  *   COPILOT_MODEL                       — model override (optional)
  *   GH_AW_COPILOT_SDK_PROVIDER_BASE_URL — BYOK provider base URL (set by the harness)
  *   GH_AW_COPILOT_SDK_PROVIDER_TYPE    — BYOK provider type: "openai" | "azure" | "anthropic" (set by the harness)
+ *   GH_AW_COPILOT_SDK_PROVIDER_WIRE_API — BYOK provider wire API: "completions" | "responses" (set by the harness)
  *   GH_AW_COPILOT_SDK_SERVER_ARGS       — JSON-encoded allow-tool sidecar args (set by the engine)
  *
  * The sidecar is started and stopped by the harness; the driver only opens a
@@ -90,8 +91,8 @@ async function main() {
 
   // --- Resolve BYOK custom provider from environment ------------------
   // The harness resolves the BYOK provider from live AWF reflect data before launching
-  // this driver and injects the result as GH_AW_COPILOT_SDK_PROVIDER_BASE_URL and
-  // GH_AW_COPILOT_SDK_PROVIDER_TYPE.
+  // this driver and injects the result as GH_AW_COPILOT_SDK_PROVIDER_BASE_URL,
+  // GH_AW_COPILOT_SDK_PROVIDER_TYPE, and GH_AW_COPILOT_SDK_PROVIDER_WIRE_API.
   // BYOK is the only supported mode — fail immediately if the base URL is missing.
   const providerBaseUrl = process.env.GH_AW_COPILOT_SDK_PROVIDER_BASE_URL;
   if (!providerBaseUrl) {
@@ -102,8 +103,14 @@ async function main() {
   /** @type {"openai" | "azure" | "anthropic"} */
   const providerType = rawProviderType === "anthropic" || rawProviderType === "azure" ? rawProviderType : "openai";
   log(`provider type: ${providerType}`);
+  const rawWireApi = process.env.GH_AW_COPILOT_SDK_PROVIDER_WIRE_API || "";
+  /** @type {"completions" | "responses" | undefined} */
+  const wireApi = rawWireApi === "responses" ? "responses" : rawWireApi === "completions" ? "completions" : undefined;
   /** @type {import("@github/copilot-sdk").ProviderConfig} */
-  const provider = { type: providerType, baseUrl: providerBaseUrl };
+  const provider = { type: providerType, baseUrl: providerBaseUrl, ...(wireApi && { wireApi }) };
+  if (wireApi) {
+    log(`provider wire API: ${wireApi}`);
+  }
 
   // --- Build permission config from sidecar server args ----------------
   // GH_AW_COPILOT_SDK_SERVER_ARGS holds the JSON-encoded --allow-tool flags
diff --git a/actions/setup/js/models.json b/actions/setup/js/models.json
index 394b698e00d..fbdc8ade0bc 100644
--- a/actions/setup/js/models.json
+++ b/actions/setup/js/models.json
@@ -287,7 +287,8 @@
             "output": "3e-05",
             "cache_read": "5e-07"
           },
-          "provider_type": "openai"
+          "provider_type": "openai",
+          "wire_api": "responses"
         },
         "claude-opus-4.5": {
           "cost": {
@@ -580,7 +581,8 @@
             "output": "3e-05",
             "cache_read": "5e-07"
           },
-          "provider_type": "openai"
+          "provider_type": "openai",
+          "wire_api": "responses"
         },
         "gpt-5.2-chat-latest": {
           "cost": {
@@ -693,7 +695,8 @@
             "input": "3e-05",
             "output": "0.00018"
           },
-          "provider_type": "openai"
+          "provider_type": "openai",
+          "wire_api": "responses"
         },
         "o4-mini-deep-research": {
           "cost": {