diff --git a/packages/core/src/v1/config/agent.ts b/packages/core/src/v1/config/agent.ts
index b220bd7ef87d..d88df65b7f83 100644
--- a/packages/core/src/v1/config/agent.ts
+++ b/packages/core/src/v1/config/agent.ts
@@ -35,6 +35,10 @@ const AgentSchema = Schema.StructWithRest(
       description: "Maximum number of agentic iterations before forcing text-only response",
     }),
     maxSteps: Schema.optional(PositiveInt).annotate({ description: "@deprecated Use 'steps' field instead." }),
+    maxStepsMessage: Schema.optional(Schema.Boolean).annotate({
+      description:
+        "Append the 'maximum steps reached' assistant message on the final step (default: true). Set to false for models that reject a trailing assistant message, such as Claude 4.6+.",
+    }),
     permission: Schema.optional(ConfigPermissionV1.Info),
   }),
   [Schema.Record(Schema.String, Schema.Any)],
@@ -53,6 +57,7 @@ const KNOWN_KEYS = new Set([
   "color",
   "steps",
   "maxSteps",
+  "maxStepsMessage",
   "options",
   "permission",
   "disable",
diff --git a/packages/opencode/src/agent/agent.ts b/packages/opencode/src/agent/agent.ts
index b1430314fffe..07b99ae4ea68 100644
--- a/packages/opencode/src/agent/agent.ts
+++ b/packages/opencode/src/agent/agent.ts
@@ -52,6 +52,7 @@ export const Info = Schema.Struct({
   prompt: Schema.optional(Schema.String),
   options: Schema.Record(Schema.String, Schema.Unknown),
   steps: Schema.optional(Schema.Finite),
+  maxStepsMessage: Schema.optional(Schema.Boolean),
 }).annotate({ identifier: "Agent" })
 export type Info = DeepMutable<Schema.Schema.Type<typeof Info>>
 
@@ -275,6 +276,7 @@ export const layer = Layer.effect(
               permission: Permission.merge(defaults, user),
               options: {},
               native: false,
+              maxStepsMessage: true,
             }
           if (value.model) item.model = Provider.parseModel(value.model)
           item.variant = value.variant ?? item.variant
@@ -287,10 +289,12 @@ export const layer = Layer.effect(
           item.hidden = value.hidden ?? item.hidden
           item.name = value.name ?? item.name
           item.steps = value.steps ?? item.steps
+          item.maxStepsMessage = value.maxStepsMessage ?? item.maxStepsMessage
           item.options = mergeDeep(item.options, value.options ?? {})
           item.permission = Permission.merge(item.permission, Permission.fromConfig(value.permission ?? {}))
         }
 
+        for (const a of Object.values(agents)) a.maxStepsMessage ??= true
         // Ensure Truncate.GLOB is allowed unless explicitly configured
         for (const name in agents) {
           const agent = agents[name]
diff --git a/packages/opencode/src/session/prompt.ts b/packages/opencode/src/session/prompt.ts
index b3f85c813f20..6d4f4d854ece 100644
--- a/packages/opencode/src/session/prompt.ts
+++ b/packages/opencode/src/session/prompt.ts
@@ -1340,7 +1340,7 @@ export const layer = Layer.effect(
               sessionID,
               parentSessionID: session.parentID,
               system,
-              messages: [...modelMsgs, ...(isLastStep ? [{ role: "assistant" as const, content: MAX_STEPS }] : [])],
+              messages: [...modelMsgs, ...(isLastStep && agent.maxStepsMessage !== false ? [{ role: "assistant" as const, content: MAX_STEPS }] : [])],
               tools,
               model,
               toolChoice: format.type === "json_schema" ? "required" : undefined,
diff --git a/packages/opencode/test/agent/agent.test.ts b/packages/opencode/test/agent/agent.test.ts
index 1df95b5c0f87..818e3f1c11d2 100644
--- a/packages/opencode/test/agent/agent.test.ts
+++ b/packages/opencode/test/agent/agent.test.ts
@@ -322,6 +322,24 @@ it.instance(
   },
 )
 
+it.instance(
+  "agent maxStepsMessage config sets maxStepsMessage property",
+  () =>
+    Effect.gen(function* () {
+      const build = yield* load((svc) => svc.get("build"))
+      const plan = yield* load((svc) => svc.get("plan"))
+      expect(build?.maxStepsMessage).toBe(false)
+      expect(plan?.maxStepsMessage).toBe(true)
+    }),
+  {
+    config: {
+      agent: {
+        build: { maxStepsMessage: false },
+      },
+    },
+  },
+)
+
 it.instance(
   "agent mode can be overridden",
   () =>
diff --git a/packages/opencode/test/session/prompt.test.ts b/packages/opencode/test/session/prompt.test.ts
index 08828018a4a3..ff8dd1019d81 100644
--- a/packages/opencode/test/session/prompt.test.ts
+++ b/packages/opencode/test/session/prompt.test.ts
@@ -791,6 +791,56 @@ it.instance("loop continues when finish is stop but assistant has tool parts", (
   }),
 )
 
+it.instance("appends MAXIMUM STEPS REACHED message on the final step by default", () =>
+  Effect.gen(function* () {
+    const { llm } = yield* useServerConfig((url) => ({
+      ...providerCfg(url),
+      agent: { build: { steps: 1 } },
+    }))
+    const prompt = yield* SessionPrompt.Service
+    const sessions = yield* Session.Service
+    const session = yield* sessions.create({
+      title: "Pinned",
+      permission: [{ permission: "*", pattern: "*", action: "allow" }],
+    })
+    yield* prompt.prompt({
+      sessionID: session.id,
+      agent: "build",
+      noReply: true,
+      parts: [{ type: "text", text: "hello" }],
+    })
+    yield* llm.text("done")
+    yield* prompt.loop({ sessionID: session.id })
+    const inputs = yield* llm.inputs
+    expect(JSON.stringify(inputs)).toContain("MAXIMUM STEPS REACHED")
+  }),
+)
+
+it.instance("omits MAXIMUM STEPS REACHED message when maxStepsMessage is false", () =>
+  Effect.gen(function* () {
+    const { llm } = yield* useServerConfig((url) => ({
+      ...providerCfg(url),
+      agent: { build: { steps: 1, maxStepsMessage: false } },
+    }))
+    const prompt = yield* SessionPrompt.Service
+    const sessions = yield* Session.Service
+    const session = yield* sessions.create({
+      title: "Pinned",
+      permission: [{ permission: "*", pattern: "*", action: "allow" }],
+    })
+    yield* prompt.prompt({
+      sessionID: session.id,
+      agent: "build",
+      noReply: true,
+      parts: [{ type: "text", text: "hello" }],
+    })
+    yield* llm.text("done")
+    yield* prompt.loop({ sessionID: session.id })
+    const inputs = yield* llm.inputs
+    expect(JSON.stringify(inputs)).not.toContain("MAXIMUM STEPS REACHED")
+  }),
+)
+
 it.instance("failed subtask preserves metadata on error tool state", () =>
   Effect.gen(function* () {
     const { llm } = yield* useServerConfig((url) => ({