diff --git a/src/clients/databricks.js b/src/clients/databricks.js
index 5d31c79..92f4ac4 100644
--- a/src/clients/databricks.js
+++ b/src/clients/databricks.js
@@ -1104,6 +1104,64 @@ async function invokeLMStudio(body) {
   return performJsonRequest(endpoint, { headers, body: lmstudioBody }, "LM Studio");
 }
 
+/**
+ * Flatten an Anthropic-style content value into a plain string for the
+ * Bedrock Converse API.
+ *
+ * Prompt-cache injection (injectPromptCaching) rewrites string `system`
+ * fields and message `content` into arrays of `{ type, text, cache_control }`
+ * blocks. The Converse API has no `cache_control` concept and expects
+ * `system: [{ text: "<string>" }]` and message content blocks shaped as
+ * `{ text: "<string>" }`. Passing the injected array through unchanged would
+ * either drop the cache markers silently or nest an array under `text`,
+ * producing a ValidationException.
+ *
+ * @param {string|Array|undefined} value - String or array of content blocks
+ * @returns {string} Concatenated plain text
+ */
+function flattenContentToText(value) {
+  if (value == null) return "";
+  if (typeof value === "string") return value;
+  if (Array.isArray(value)) {
+    return value
+      .map(block => {
+        if (typeof block === "string") return block;
+        if (block && typeof block === "object") return block.text || block.content || "";
+        return "";
+      })
+      .join("");
+  }
+  return String(value);
+}
+
+/**
+ * Normalize a request body for the Bedrock Converse API.
+ *
+ * Strips `cache_control` markers and flattens any array-shaped `system` /
+ * message `content` (left behind by prompt-cache injection) back into the
+ * plain strings the Converse API expects. Returns a shallow copy with a
+ * normalized `messages` array; the original body is not mutated.
+ *
+ * @param {Object} body - Anthropic-format request body
+ * @returns {Object} Body safe for Converse request construction
+ */
+function normalizeBodyForConverse(body) {
+  const normalized = { ...body };
+
+  if (normalized.system !== undefined) {
+    normalized.system = flattenContentToText(normalized.system);
+  }
+
+  if (Array.isArray(normalized.messages)) {
+    normalized.messages = normalized.messages.map(msg => ({
+      ...msg,
+      content: flattenContentToText(msg.content),
+    }));
+  }
+
+  return normalized;
+}
+
 async function invokeBedrock(body) {
   // 1. Validate Bearer token
   if (!config.bedrock?.apiKey) {
@@ -1130,7 +1188,10 @@ async function invokeBedrock(body) {
     }, "=== INJECTING STANDARD TOOLS (Bedrock) ===");
   }
 
-  const bedrockBody = { ...body, tools: toolsToSend };
+  // Normalize away cache_control / array shapes that prompt-cache injection
+  // may have applied: the Converse API expects plain-string system and
+  // message content, not Anthropic cache_control blocks.
+  const bedrockBody = { ...normalizeBodyForConverse(body), tools: toolsToSend };
 
   // 4. Detect model family and convert format
   const modelId = body._tierModel || config.bedrock.modelId;
@@ -2579,4 +2640,5 @@ function destroyHttpAgents() {
 module.exports = {
   invokeModel,
   destroyHttpAgents,
+  normalizeBodyForConverse,
 };
diff --git a/src/clients/prompt-cache-injection.js b/src/clients/prompt-cache-injection.js
index d0460bb..e1e774e 100644
--- a/src/clients/prompt-cache-injection.js
+++ b/src/clients/prompt-cache-injection.js
@@ -119,6 +119,51 @@ function needsCacheInjection(provider) {
   return EXPLICIT_CACHE_PROVIDERS.has(provider);
 }
 
+// Model families that do NOT support cache_control breakpoints. cache_control
+// is an Anthropic construct; on aggregating providers (Bedrock, OpenRouter) it
+// only applies to models that natively understand it (Claude, and Gemini via
+// proxy). Injecting markers onto these families produces request shapes the
+// upstream model rejects or silently ignores.
+const NON_CACHE_MODEL_PATTERNS = [
+  /(^|[./-])titan/i,
+  /(^|[./-])nova/i,
+  /(^|[./-])llama/i,
+  /(^|[./-])mistral/i,
+  /(^|[./-])mixtral/i,
+  /(^|[./-])cohere/i,
+  /(^|[./-])command/i, // cohere command-*
+  /(^|[./-])j2/i,      // ai21 jurassic
+  /(^|[./-])jamba/i,
+  /(^|[./-])deepseek/i,
+  /(^|[./-])qwen/i,
+  /(^|[./-])gpt/i,
+  /(^|[./-])openai/i,
+];
+
+/**
+ * Determine whether the model targeted by this request supports cache_control.
+ *
+ * Some providers in EXPLICIT_CACHE_PROVIDERS (notably bedrock and openrouter)
+ * route to many model families, only some of which understand cache_control.
+ * This guard inspects the resolved model id and blocks injection for families
+ * that are known not to support it. When the model id is absent or
+ * unrecognized, injection is allowed (fail-open) — Claude/Gemini-style ids and
+ * Anthropic-only providers fall through to true.
+ *
+ * @param {Object} body - Request body (may carry the resolved model id)
+ * @param {string} provider - Provider name
+ * @returns {boolean}
+ */
+function modelSupportsCacheControl(body, provider) {
+  // Providers that only ever route to Anthropic models always support it.
+  if (provider === 'azure-anthropic' || provider === 'databricks') return true;
+
+  const modelId = body && (body._tierModel || body.model);
+  if (!modelId || typeof modelId !== 'string') return true; // unknown → fail open
+
+  return !NON_CACHE_MODEL_PATTERNS.some(re => re.test(modelId));
+}
+
 /**
  * Inject provider-side prompt caching into the request body.
  * Call this before sending to the provider.
@@ -129,6 +174,9 @@ function needsCacheInjection(provider) {
  */
 function injectPromptCaching(body, provider) {
   if (!needsCacheInjection(provider)) return 0;
+  // Gate on model capability: a provider may support cache_control in general
+  // while the specific routed model does not.
+  if (!modelSupportsCacheControl(body, provider)) return 0;
   return injectAnthropicCacheBreakpoints(body);
 }
 
@@ -137,4 +185,5 @@ module.exports = {
   injectAnthropicCacheBreakpoints,
   injectGeminiCacheBreakpoints,
   needsCacheInjection,
+  modelSupportsCacheControl,
 };
diff --git a/test/databricks-bedrock-converse.test.js b/test/databricks-bedrock-converse.test.js
new file mode 100644
index 0000000..890bb8d
--- /dev/null
+++ b/test/databricks-bedrock-converse.test.js
@@ -0,0 +1,134 @@
+/**
+ * Tests for the Bedrock Converse request normalization.
+ *
+ * invokeModel runs injectPromptCaching() before dispatching to a provider,
+ * which rewrites string `system` / message `content` into Anthropic
+ * cache_control blocks. The Bedrock Converse API has no cache_control
+ * concept and requires plain-string system and message content, so the
+ * Bedrock path must flatten those shapes back before building the request.
+ */
+
+const assert = require("assert");
+const { describe, it, beforeEach, afterEach } = require("node:test");
+
+const { injectPromptCaching } = require("../src/clients/prompt-cache-injection");
+
+describe("Bedrock Converse normalization", () => {
+  let originalEnv;
+  let normalizeBodyForConverse;
+
+  beforeEach(() => {
+    originalEnv = { ...process.env };
+    // databricks.js loads ../config at require time; give it valid creds.
+    process.env.MODEL_PROVIDER = "databricks";
+    process.env.DATABRICKS_API_KEY = "test-key";
+    process.env.DATABRICKS_API_BASE = "http://test.com";
+
+    delete require.cache[require.resolve("../src/config")];
+    delete require.cache[require.resolve("../src/clients/databricks")];
+    ({ normalizeBodyForConverse } = require("../src/clients/databricks"));
+  });
+
+  afterEach(() => {
+    process.env = originalEnv;
+  });
+
+  it("flattens a cache_control-injected system prompt back to a string", () => {
+    const body = {
+      system: "You are a helpful assistant",
+      messages: [{ role: "user", content: "Hello" }],
+    };
+
+    // Simulate the injection that invokeModel performs before dispatch.
+    injectPromptCaching(body, "bedrock");
+    assert.ok(Array.isArray(body.system), "precondition: injection made system an array");
+
+    const normalized = normalizeBodyForConverse(body);
+
+    assert.strictEqual(typeof normalized.system, "string");
+    assert.strictEqual(normalized.system, "You are a helpful assistant");
+  });
+
+  it("flattens injected message content back to plain strings", () => {
+    const body = {
+      system: "sys",
+      messages: [
+        { role: "user", content: "first" },
+        { role: "assistant", content: "second" },
+        { role: "user", content: "third" },
+      ],
+    };
+
+    injectPromptCaching(body, "bedrock");
+
+    const normalized = normalizeBodyForConverse(body);
+
+    for (const msg of normalized.messages) {
+      assert.strictEqual(typeof msg.content, "string");
+    }
+    assert.strictEqual(normalized.messages[0].content, "first");
+    assert.strictEqual(normalized.messages[2].content, "third");
+  });
+
+  it("produces a Converse-valid system shape ([{text:string}])", () => {
+    const body = {
+      system: "cached system prompt",
+      messages: [{ role: "user", content: "hi" }],
+    };
+
+    injectPromptCaching(body, "bedrock");
+    const normalized = normalizeBodyForConverse(body);
+
+    // Mirror how invokeBedrock builds the Converse system field.
+    const converseSystem = [{ text: normalized.system }];
+    assert.strictEqual(converseSystem.length, 1);
+    assert.strictEqual(typeof converseSystem[0].text, "string");
+    assert.strictEqual(converseSystem[0].text, "cached system prompt");
+  });
+
+  it("produces Converse-valid content blocks ({text:string})", () => {
+    const body = {
+      system: "sys",
+      messages: [{ role: "user", content: "question" }],
+    };
+
+    injectPromptCaching(body, "bedrock");
+    const normalized = normalizeBodyForConverse(body);
+
+    // Mirror how invokeBedrock maps message content for Converse.
+    const blocks = Array.isArray(normalized.messages[0].content)
+      ? normalized.messages[0].content.map(c => ({ text: c.text || c.content || "" }))
+      : [{ text: normalized.messages[0].content }];
+
+    assert.strictEqual(blocks.length, 1);
+    assert.strictEqual(typeof blocks[0].text, "string");
+    assert.strictEqual(blocks[0].text, "question");
+  });
+
+  it("does not strip cache_control from the caller's original body", () => {
+    const body = {
+      system: "sys",
+      messages: [{ role: "user", content: "hi" }],
+    };
+
+    injectPromptCaching(body, "bedrock");
+    normalizeBodyForConverse(body);
+
+    // Normalization works on a copy, leaving the injected body untouched.
+    assert.ok(Array.isArray(body.system));
+    assert.deepStrictEqual(body.system[0].cache_control, { type: "ephemeral" });
+  });
+
+  it("handles array system blocks without cache_control", () => {
+    const body = {
+      system: [
+        { type: "text", text: "Part A" },
+        { type: "text", text: "Part B" },
+      ],
+      messages: [],
+    };
+
+    const normalized = normalizeBodyForConverse(body);
+    assert.strictEqual(normalized.system, "Part APart B");
+  });
+});
diff --git a/test/prompt-cache-injection.test.js b/test/prompt-cache-injection.test.js
index 753ce5a..b9b6b64 100644
--- a/test/prompt-cache-injection.test.js
+++ b/test/prompt-cache-injection.test.js
@@ -9,6 +9,7 @@ const {
   injectAnthropicCacheBreakpoints,
   injectPromptCaching,
   needsCacheInjection,
+  modelSupportsCacheControl,
 } = require('../src/clients/prompt-cache-injection');
 
 // ── needsCacheInjection ─────────────────────────────────────────────
@@ -220,3 +221,82 @@ describe('injectPromptCaching', () => {
     assert.equal(count, 0);
   });
 });
+
+// ── model-capability gate ───────────────────────────────────────────
+
+describe('modelSupportsCacheControl', () => {
+  it('always supports for Anthropic-only providers', () => {
+    assert.equal(modelSupportsCacheControl({}, 'azure-anthropic'), true);
+    assert.equal(modelSupportsCacheControl({ _tierModel: 'whatever' }, 'databricks'), true);
+  });
+
+  it('fails open when no model id is present', () => {
+    assert.equal(modelSupportsCacheControl({}, 'bedrock'), true);
+    assert.equal(modelSupportsCacheControl({}, 'openrouter'), true);
+  });
+
+  it('supports Claude model ids on bedrock', () => {
+    assert.equal(
+      modelSupportsCacheControl(
+        { _tierModel: 'us.anthropic.claude-sonnet-4-5-20250929-v1:0' },
+        'bedrock'
+      ),
+      true
+    );
+  });
+
+  it('blocks non-Claude bedrock families', () => {
+    assert.equal(modelSupportsCacheControl({ _tierModel: 'meta.llama3-70b-instruct-v1:0' }, 'bedrock'), false);
+    assert.equal(modelSupportsCacheControl({ _tierModel: 'amazon.titan-text-express-v1' }, 'bedrock'), false);
+    assert.equal(modelSupportsCacheControl({ _tierModel: 'mistral.mistral-7b-instruct-v0:2' }, 'bedrock'), false);
+    assert.equal(modelSupportsCacheControl({ _tierModel: 'cohere.command-text-v14' }, 'bedrock'), false);
+  });
+
+  it('blocks non-supporting openrouter models', () => {
+    assert.equal(modelSupportsCacheControl({ model: 'meta-llama/llama-3-70b' }, 'openrouter'), false);
+    assert.equal(modelSupportsCacheControl({ model: 'openai/gpt-4o' }, 'openrouter'), false);
+  });
+});
+
+describe('injectPromptCaching capability gate', () => {
+  it('still injects for bedrock when the model id is unknown (backward compatible)', () => {
+    const body = {
+      system: 'test',
+      messages: [{ role: 'user', content: 'hi' }],
+    };
+    const count = injectPromptCaching(body, 'bedrock');
+    assert.equal(count, 2);
+  });
+
+  it('injects for a Claude model on bedrock', () => {
+    const body = {
+      _tierModel: 'anthropic.claude-3-5-sonnet-20241022-v2:0',
+      system: 'test',
+      messages: [{ role: 'user', content: 'hi' }],
+    };
+    const count = injectPromptCaching(body, 'bedrock');
+    assert.equal(count, 2);
+  });
+
+  it('skips injection for a non-Claude bedrock model and leaves body untouched', () => {
+    const body = {
+      _tierModel: 'meta.llama3-70b-instruct-v1:0',
+      system: 'test',
+      messages: [{ role: 'user', content: 'hi' }],
+    };
+    const count = injectPromptCaching(body, 'bedrock');
+    assert.equal(count, 0);
+    assert.equal(body.system, 'test'); // unchanged string, no array conversion
+    assert.equal(body.messages[0].content, 'hi');
+  });
+
+  it('skips injection for a GPT model routed via openrouter', () => {
+    const body = {
+      model: 'openai/gpt-4o',
+      system: 'test',
+      messages: [{ role: 'user', content: 'hi' }],
+    };
+    const count = injectPromptCaching(body, 'openrouter');
+    assert.equal(count, 0);
+  });
+});