diff --git a/src/harness/providers/openai/mod.rs b/src/harness/providers/openai/mod.rs
index 04568c7..9e0a697 100644
--- a/src/harness/providers/openai/mod.rs
+++ b/src/harness/providers/openai/mod.rs
@@ -66,6 +66,7 @@ const DEFAULT_CONNECT_TIMEOUT_SECS: u64 = 30;
 const DEFAULT_REQUEST_TIMEOUT_SECS: u64 = 600;
 
 mod convert;
+mod prompt_tools;
 mod responses;
 mod sse;
 mod transport;
diff --git a/src/harness/providers/openai/prompt_tools.rs b/src/harness/providers/openai/prompt_tools.rs
new file mode 100644
index 0000000..a830e61
--- /dev/null
+++ b/src/harness/providers/openai/prompt_tools.rs
@@ -0,0 +1,250 @@
+//! Prompt-guided (text-mode) tool calling for models without native tool support.
+//!
+//! Many self-hosted / local OpenAI-compatible runtimes reject the OpenAI `tools`
+//! parameter (HTTP 400) or simply ignore it. For those models
+//! ([`OpenAiModel`](super::OpenAiModel) built [`with_native_tool_calling(false)`](super::OpenAiModel::with_native_tool_calling)),
+//! the wire client embeds the tool specs **in the system prompt** as a small
+//! protocol and parses the model's `<tool_call>…</tool_call>` blocks back into
+//! [`ToolCall`]s — so the agent loop sees tool calls identically to the native
+//! path, without any harness change.
+//!
+//! The `<tool_call>{"name":…,"arguments":…}</tool_call>` convention matches the
+//! long-standing OpenHuman host format so models already prompted for it behave
+//! identically after the crate cutover.
+
+use std::fmt::Write as _;
+
+use serde_json::{Map, Value};
+
+use crate::harness::message::{ContentBlock, Message};
+use crate::harness::model::ModelResponse;
+use crate::harness::tool::{ToolCall, ToolSchema};
+
+/// Opening / closing delimiters for a text-mode tool call.
+const OPEN_TAG: &str = "<tool_call>";
+const CLOSE_TAG: &str = "</tool_call>";
+
+/// Build the tool-use protocol block appended to the system prompt when native
+/// tool calling is unavailable. Describes the `<tool_call>` convention and lists
+/// each tool's name, description, and JSON-Schema parameters.
+pub(super) fn tool_instructions(tools: &[ToolSchema]) -> String {
+    let mut out = String::new();
+    out.push_str("## Tool Use Protocol\n\n");
+    out.push_str("To use a tool, wrap a JSON object in <tool_call></tool_call> tags:\n\n");
+    out.push_str(OPEN_TAG);
+    out.push('\n');
+    out.push_str(r#"{"name": "tool_name", "arguments": {"param": "value"}}"#);
+    out.push('\n');
+    out.push_str(CLOSE_TAG);
+    out.push_str("\n\n");
+    out.push_str("You may emit multiple tool calls in a single response. ");
+    out.push_str("After execution, results appear in <tool_result> tags. ");
+    out.push_str("Continue reasoning with the results until you can give a final answer.\n\n");
+    out.push_str("### Available Tools\n\n");
+    for tool in tools {
+        let params = serde_json::to_string(&tool.parameters).unwrap_or_else(|_| "{}".to_string());
+        // Infallible: writing to a String never errors.
+        let _ = writeln!(out, "**{}**: {}", tool.name, tool.description);
+        let _ = writeln!(out, "Parameters: `{params}`\n");
+    }
+    out
+}
+
+/// Return `messages` with the tool-use protocol appended to the system prompt:
+/// the instructions are added as a trailing block on the first system message, or
+/// a new leading system message when the request carries none. `tools` empty →
+/// `messages` is returned unchanged (cloned).
+pub(super) fn with_tool_instructions(messages: &[Message], tools: &[ToolSchema]) -> Vec<Message> {
+    if tools.is_empty() {
+        return messages.to_vec();
+    }
+    let block = tool_instructions(tools);
+    let mut out = messages.to_vec();
+    if let Some(Message::System(system)) = out.iter_mut().find(|m| matches!(m, Message::System(_)))
+    {
+        // Append as a distinct text block so the original system prompt is intact.
+        system
+            .content
+            .push(ContentBlock::Text(format!("\n\n{block}")));
+    } else {
+        out.insert(0, Message::system(block));
+    }
+    out
+}
+
+/// Extract `<tool_call>…</tool_call>` blocks from `text`, parsing each inner JSON
+/// object (`{"name":…,"arguments":…}`) into a [`ToolCall`]. Returns the text with
+/// the blocks removed (trimmed) plus the parsed calls, in order.
+///
+/// Robust to noise: a block whose inner text is not a JSON object with a string
+/// `name` is dropped; a dangling `<tool_call>` with no close is left verbatim in
+/// the returned text.
+pub(super) fn parse_tool_calls_from_text(text: &str) -> (String, Vec<ToolCall>) {
+    let mut calls = Vec::new();
+    let mut cleaned = String::new();
+    let mut rest = text;
+
+    while let Some(start) = rest.find(OPEN_TAG) {
+        cleaned.push_str(&rest[..start]);
+        let after_open = &rest[start + OPEN_TAG.len()..];
+        let Some(end) = after_open.find(CLOSE_TAG) else {
+            // Unterminated block: keep it (and everything after) as plain text.
+            cleaned.push_str(&rest[start..]);
+            return (cleaned.trim().to_string(), calls);
+        };
+        let inner = after_open[..end].trim();
+        if let Some(call) = parse_one(inner, calls.len() + 1) {
+            calls.push(call);
+        }
+        rest = &after_open[end + CLOSE_TAG.len()..];
+    }
+    cleaned.push_str(rest);
+    (cleaned.trim().to_string(), calls)
+}
+
+/// Extract prompt-guided `<tool_call>` blocks from a completed [`ModelResponse`]'s
+/// text into `message.tool_calls`, replacing the message content with the cleaned
+/// prose. No-op when the text carries no blocks — so a plain text answer is
+/// untouched. Applied by [`OpenAiModel`](super::OpenAiModel) after a non-native
+/// tool call completes (unary + terminal stream item).
+pub(super) fn apply_to_response(mut response: ModelResponse) -> ModelResponse {
+    let text = response.text();
+    let (cleaned, calls) = parse_tool_calls_from_text(&text);
+    if calls.is_empty() {
+        return response;
+    }
+    response.message.tool_calls.extend(calls);
+    response.message.content = if cleaned.is_empty() {
+        Vec::new()
+    } else {
+        vec![ContentBlock::Text(cleaned)]
+    };
+    response
+}
+
+/// Parse a single tool-call body into a [`ToolCall`] with a synthetic 1-based id.
+fn parse_one(inner: &str, index: usize) -> Option<ToolCall> {
+    let value: Value = serde_json::from_str(inner).ok()?;
+    let name = value.get("name")?.as_str()?.to_string();
+    let arguments = value
+        .get("arguments")
+        .cloned()
+        .unwrap_or_else(|| Value::Object(Map::new()));
+    Some(ToolCall {
+        id: format!("call_{index}"),
+        name,
+        arguments,
+    })
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    fn schema(name: &str) -> ToolSchema {
+        ToolSchema {
+            name: name.to_string(),
+            description: format!("{name} description"),
+            parameters: serde_json::json!({"type": "object"}),
+            format: Default::default(),
+        }
+    }
+
+    #[test]
+    fn instructions_list_each_tool() {
+        let text = tool_instructions(&[schema("read_file"), schema("write_file")]);
+        assert!(text.contains("## Tool Use Protocol"));
+        assert!(text.contains("<tool_call>"));
+        assert!(text.contains("**read_file**"));
+        assert!(text.contains("**write_file**"));
+    }
+
+    #[test]
+    fn with_tool_instructions_appends_to_system() {
+        let msgs = vec![Message::system("You are helpful."), Message::user("hi")];
+        let out = with_tool_instructions(&msgs, &[schema("read_file")]);
+        assert_eq!(out.len(), 2);
+        let Message::System(sys) = &out[0] else {
+            panic!("first message should stay system")
+        };
+        // Original prompt preserved + protocol appended.
+        let joined: String = sys
+            .content
+            .iter()
+            .filter_map(|b| match b {
+                ContentBlock::Text(t) => Some(t.as_str()),
+                _ => None,
+            })
+            .collect();
+        assert!(joined.contains("You are helpful."));
+        assert!(joined.contains("Tool Use Protocol"));
+    }
+
+    #[test]
+    fn with_tool_instructions_inserts_system_when_absent() {
+        let msgs = vec![Message::user("hi")];
+        let out = with_tool_instructions(&msgs, &[schema("read_file")]);
+        assert_eq!(out.len(), 2);
+        assert!(matches!(out[0], Message::System(_)));
+    }
+
+    #[test]
+    fn empty_tools_leaves_messages_unchanged() {
+        let msgs = vec![Message::user("hi")];
+        assert_eq!(with_tool_instructions(&msgs, &[]), msgs);
+    }
+
+    #[test]
+    fn parses_single_tool_call() {
+        let text = r#"Let me read it.
+<tool_call>
+{"name": "read_file", "arguments": {"path": "a.txt"}}
+</tool_call>"#;
+        let (cleaned, calls) = parse_tool_calls_from_text(text);
+        assert_eq!(cleaned, "Let me read it.");
+        assert_eq!(calls.len(), 1);
+        assert_eq!(calls[0].name, "read_file");
+        assert_eq!(calls[0].id, "call_1");
+        assert_eq!(calls[0].arguments, serde_json::json!({"path": "a.txt"}));
+    }
+
+    #[test]
+    fn parses_multiple_calls_and_keeps_prose() {
+        let text = r#"a<tool_call>{"name":"one","arguments":{}}</tool_call>b<tool_call>{"name":"two","arguments":{"x":1}}</tool_call>c"#;
+        let (cleaned, calls) = parse_tool_calls_from_text(text);
+        assert_eq!(cleaned, "abc");
+        assert_eq!(calls.len(), 2);
+        assert_eq!(calls[0].name, "one");
+        assert_eq!(calls[1].name, "two");
+        assert_eq!(calls[1].id, "call_2");
+    }
+
+    #[test]
+    fn missing_arguments_defaults_to_empty_object() {
+        let (_, calls) = parse_tool_calls_from_text(r#"<tool_call>{"name":"noargs"}</tool_call>"#);
+        assert_eq!(calls.len(), 1);
+        assert_eq!(calls[0].arguments, serde_json::json!({}));
+    }
+
+    #[test]
+    fn malformed_block_is_dropped() {
+        let (cleaned, calls) = parse_tool_calls_from_text("<tool_call>not json</tool_call>done");
+        assert!(calls.is_empty());
+        assert_eq!(cleaned, "done");
+    }
+
+    #[test]
+    fn unterminated_block_kept_as_text() {
+        let text = "text <tool_call>{\"name\":\"x\"}";
+        let (cleaned, calls) = parse_tool_calls_from_text(text);
+        assert!(calls.is_empty());
+        assert_eq!(cleaned, "text <tool_call>{\"name\":\"x\"}");
+    }
+
+    #[test]
+    fn no_blocks_returns_text_verbatim() {
+        let (cleaned, calls) = parse_tool_calls_from_text("just a normal answer");
+        assert!(calls.is_empty());
+        assert_eq!(cleaned, "just a normal answer");
+    }
+}
diff --git a/src/harness/providers/openai/transport.rs b/src/harness/providers/openai/transport.rs
index 92a569f..9e8d71a 100644
--- a/src/harness/providers/openai/transport.rs
+++ b/src/harness/providers/openai/transport.rs
@@ -685,32 +685,50 @@ impl OpenAiModel {
         &self,
         request: &ModelRequest,
     ) -> Result<ChatCompletionRequest> {
+        // Prompt-guided tools: a model without native tool calling that is still
+        // handed tools gets the tool protocol embedded in its system prompt and no
+        // native `tools` on the wire (many local runtimes 400 on `tools`). The
+        // model's `<tool_call>` blocks are parsed back in [`Self::invoke`]/stream.
+        let prompt_guided_tools = !self.profile.tool_calling && !request.tools.is_empty();
+        let instructed_messages;
+        let base_messages: &[Message] = if prompt_guided_tools {
+            instructed_messages =
+                prompt_tools::with_tool_instructions(&request.messages, &request.tools);
+            &instructed_messages
+        } else {
+            &request.messages
+        };
+
         // Optionally fold system messages into the first user turn (for endpoints
         // that reject a `system` role) before wire translation.
         let merged_messages;
         let source_messages: &[Message] = if self.merge_system_into_user {
-            merged_messages = merge_system_into_user(&request.messages);
+            merged_messages = merge_system_into_user(base_messages);
             &merged_messages
         } else {
-            &request.messages
+            base_messages
         };
         let messages = source_messages
             .iter()
             .map(translate_message)
             .collect::<Result<Vec<_>>>()?;
 
-        let tools: Vec<ToolWire> = request
-            .tools
-            .iter()
-            .map(|schema| ToolWire {
-                kind: "function".to_string(),
-                function: FunctionSchemaWire {
-                    name: schema.name.clone(),
-                    description: schema.description.clone(),
-                    parameters: schema.parameters.clone(),
-                },
-            })
-            .collect();
+        let tools: Vec<ToolWire> = if prompt_guided_tools {
+            Vec::new()
+        } else {
+            request
+                .tools
+                .iter()
+                .map(|schema| ToolWire {
+                    kind: "function".to_string(),
+                    function: FunctionSchemaWire {
+                        name: schema.name.clone(),
+                        description: schema.description.clone(),
+                        parameters: schema.parameters.clone(),
+                    },
+                })
+                .collect()
+        };
 
         // tool_choice is only meaningful when tools are declared.
         let tool_choice = if tools.is_empty() {
@@ -1084,7 +1102,13 @@ impl<State: Send + Sync> ChatModel<State> for OpenAiModel {
         })?;
 
         let value: Value = serde_json::from_str(&text)?;
-        parse_response(value)
+        let response = parse_response(value)?;
+        // Prompt-guided tools: recover the model's `<tool_call>` blocks into
+        // `message.tool_calls` when native tool calling was suppressed.
+        if !self.profile.tool_calling && !request.tools.is_empty() {
+            return Ok(prompt_tools::apply_to_response(response));
+        }
+        Ok(response)
     }
 
     /// Streams the OpenAI Chat Completions response as a real [`ModelStream`].
@@ -1153,6 +1177,18 @@ impl<State: Send + Sync> ChatModel<State> for OpenAiModel {
             terminal_emitted: false,
         };
 
-        Ok(Box::pin(futures::stream::unfold(state, sse_next)))
+        let stream = futures::stream::unfold(state, sse_next);
+        // Prompt-guided tools: recover `<tool_call>` blocks from the terminal
+        // `Completed` response into `message.tool_calls` (the streamed text deltas
+        // still carry the raw markup — cleaning them mid-stream is a follow-up).
+        if !self.profile.tool_calling && !request.tools.is_empty() {
+            return Ok(Box::pin(stream.map(|item| match item {
+                ModelStreamItem::Completed(response) => {
+                    ModelStreamItem::Completed(prompt_tools::apply_to_response(response))
+                }
+                other => other,
+            })));
+        }
+        Ok(Box::pin(stream))
     }
 }